Radio frequency coil methods and apparatus

ABSTRACT

Aspects relate to providing radio frequency components responsive to magnetic resonance signals. According to some aspects, a radio frequency component comprises at least one coil having a conductor arranged in a plurality of turns oriented about a region of interest to respond to corresponding magnetic resonant signal components. According to some aspects, the radio frequency component comprises a plurality of coils oriented to respond to corresponding magnetic resonant signal components. According to some aspects, an optimization is used to determine a configuration for at least one radio frequency coil.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/926,272 filed Jul. 10, 2020, which is a continuation of U.S. patentapplication Ser. No. 15/152,951 filed May 12, 2016, which claimspriority under 35 U.S.C. § 119 to U.S. Provisional Patent ApplicationNo. 62/160,036, filed May 12, 2015, titled “RECEIVE COIL OPTIMIZATIONMETHODS AND APPARATUS,” and U.S. Provisional Patent Application No.62/169,102, filed Jun. 1, 2015, titled “COIL OPTIMIZATION METHODS ANDAPPARATUS,” each of which is hereby incorporated by reference in itsentirety.

BACKGROUND

Magnetic resonance imaging (MRI) provides an important imaging modalityfor numerous applications and is widely utilized in clinical andresearch settings to produce images of the inside of the human body. Asa generality, MRI is based on detecting magnetic resonance (MR) signals,which are electromagnetic waves emitted by atoms in response to statechanges resulting from applied electromagnetic fields. For example,nuclear magnetic resonance (NMR) techniques involve detecting MR signalsemitted from the nuclei of excited atoms upon the re-alignment orrelaxation of the nuclear spin of atoms in an object being imaged (e.g.,atoms in the tissue of the human body). Detected MR signals may beprocessed to produce images, which in the context of medicalapplications, allows for the investigation of internal structures and/orbiological processes within the body for diagnostic, therapeutic and/orresearch purposes.

MRI provides an attractive imaging modality for biological imaging dueto the ability to produce non-invasive images having relatively highresolution and contrast without the safety concerns of other modalities(e.g., without needing to expose the subject to ionizing radiation,e.g., x-rays, or introducing radioactive material to the body).Additionally, MRI is particularly well suited to provide soft tissuecontrast, which can be exploited to image subject matter that otherimaging modalities are incapable of satisfactorily imaging. Moreover, MRtechniques are capable of capturing information about structures and/orbiological processes that other modalities are incapable of acquiring.However, there are a number of drawbacks to MRI that, for a givenimaging application, may involve the relatively high cost of theequipment, limited availability and/or difficulty in gaining access toclinical MRI scanners and/or the length of the image acquisitionprocess.

The trend in clinical MRI has been to increase the field strength of MRIscanners to improve one or more of scan time, image resolution, andimage contrast, which, in turn, continues to drive up costs. The vastmajority of installed MRI scanners operate at 1.5 or 3 tesla (T), whichrefers to the field strength of the main magnetic field B₀. A rough costestimate for a clinical MRI scanner is approximately one million dollarsper tesla, which does not factor in the substantial operation, service,and maintenance costs involved in operating such MRI scanners.

Additionally, conventional high-field MRI systems typically requirelarge superconducting magnets and associated electronics to generate astrong uniform static magnetic field (B₀) in which an object (e.g., apatient) is imaged. The size of such systems is considerable with atypical MRI installment including multiple rooms for the magnet,electronics, thermal management system, and control console areas. Thesize and expense of MRI systems generally limits their usage tofacilities, such as hospitals and academic research centers, which havesufficient space and resources to purchase and maintain them. The highcost and substantial space requirements of high-field MRI systemsresults in limited availability of MRI scanners. As such, there arefrequently clinical situations in which an MRI scan would be beneficial,but due to one or more of the limitations discussed above, is notpractical or is impossible, as discussed in further detail below.

SUMMARY

The inventors have developed radio frequency components that, in someembodiments, are configured to improve magnetic resonance signaldetection to, for example, facilitate image acquisition at low fieldstrengths. Some embodiments include a radio frequency coil configured tobe responsive to magnetic resonance signals, the radio frequency coilcomprising at least one conductor arranged in a three dimensionalgeometry about a region of interest in a configuration optimized toincrease sensitivity to magnetic resonance signals emitted within theregion of interest.

Some embodiments include a radio frequency component configured to beresponsive to magnetic resonance signals, the radio frequency componentcomprising a first coil, including a first conductor arranged in aplurality of turns, oriented to be responsive to first magneticresonance signal components, and a second coil, including a secondconductor arranged in a plurality of turns, oriented to be responsive tosecond magnetic resonance signal components.

Some embodiments include a radio frequency component configured to beresponsive to magnetic resonance signals, the radio frequency componentcomprising a first coil including a first conductor arranged in aplurality of turns oriented to be responsive to magnetic resonancesignal components along a first principal axis, and a second coilincluding a second conductor arranged in a plurality of turns orientedto be responsive to magnetic resonance signal components along a secondprincipal axis oriented differently than the first principal axis.

Some embodiments include a radio frequency component configured to beresponsive to magnetic resonance signals, the radio frequency componentcomprising a first coil including a first conductor having a pluralityof turns arranged about a region of interest, and a second coilincluding a second conductor having a plurality of turns arranged aboutthe region of interest and offset from the first coil away from theregion of interest.

Some embodiments include a radio frequency coil configured to beresponsive to magnetic resonance signals, the radio frequency coilcomprising at least one conductor arranged in a three dimensionalgeometry about a region of interest, wherein a coil configuration of theat least one conductor in the three dimensional geometry is determinedbased, at least in part, on performing at least one optimization using amodel of the radio frequency coil.

Some embodiments include a method of determining a configuration for aradio-frequency coil comprising generating a model of theradio-frequency coil, and performing an optimization to determine amodel configuration that satisfies at least one constraint and that,when operation of the model is simulated, produces a magnetic field thatsatisfies a predetermined criteria.

Some embodiments include a radio frequency coil configured for a portionof a body of a patient, the radio frequency coil comprising at least oneconductor arranged in a plurality of turns about a region of interestand oriented to be responsive to magnetic resonance signal componentsoriented substantially orthogonal to a longitudinal axis of the targetanatomy of the patient.

Some embodiments include an apparatus for use in a magnetic resonanceimaging system, the apparatus comprising a first coil, and at least onecontroller configured to operate the coil to generate a radio frequencymagnetic field and a gradient field.

Some embodiments include a radio frequency coil configured for a portionof human anatomy, the radio frequency coil comprising at least oneconductor arranged in a three dimensional geometry about a region ofinterest, the at least one conductor forming a plurality of turns,wherein spacing between the plurality of turns is non-uniform.

Some embodiments include a low-field magnetic resonance systemcomprising a B0 magnet configured to produce a low-field strength B0magnetic field to provide a field of view, a first coil configured to beresponsive to first magnetic resonance signal components emitted fromthe field of view, and a second coil configured to be responsive tosecond magnetic resonance signal components emitted from the field ofview.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects and embodiments of the disclosed technology will bedescribed with reference to the following figures. It should beappreciated that the figures are not necessarily drawn to scale.

FIG. 1 illustrates a block diagram of an exemplary magnetic resonanceimaging system, in accordance with some embodiments;

FIGS. 2A and 2B illustrate bi-planar magnet geometries, in accordancewith some embodiments;

FIGS. 3A and 3B illustrate exemplary head coils, in accordance with someembodiments;

FIGS. 4A and 4B illustrate respective methods of determining aconfiguration of a radio frequency coil, in accordance with someembodiments;

FIG. 5 illustrates a method of determining a configuration of a radiofrequency coil using a model of the radio frequency coil that includes amesh, in accordance with some embodiments;

FIG. 6A illustrates an exemplary triangular mesh for use in a model ofan exemplary head coil, in accordance with some embodiments;

FIG. 6B illustrates an exemplary triangular mesh for use in a model ofan exemplary leg coil, in accordance with some embodiments;

FIG. 7A illustrates an optimized model configuration for an exemplaryhead coil, in accordance with some embodiments;

FIG. 7B illustrates an optimized model configuration for an exemplaryleg coil, in accordance with some embodiments;

FIGS. 8A and 8B illustrate an exemplary coil configuration determinedfrom the optimized model configuration illustrated in FIGS. 7A, inaccordance with some embodiments;

FIGS. 9A and 9B illustrate an exemplary coil configuration determinedfrom the optimized model configuration illustrated in FIGS. 7B, inaccordance with some embodiments;

FIGS. 10A and 10B illustrate views of a support surface having groovesto accommodate a conductor in accordance with the coil configurationillustrated in FIGS. 8A and 8B;

FIG. 11 illustrates a support surface having grooves to accommodate aconductor in accordance with the coil configuration illustrated in FIGS.9A and 9B;

FIG. 12 illustrates a method of determining a coil configuration andapplying the coil configuration to a support structure, in accordancewith some embodiments;

FIG. 13A illustrates a B0 magnet arranged in a bi-planar geometry;

FIG. 13B illustrates a B0 magnet arranged in a cylindrical geometry;

FIG. 13C illustrates a coil configuration for a head coil depicted witha set of orthogonal axes;

FIG. 13D illustrates a coil configuration for a leg coil depicted with aset of orthogonal axes;

FIGS. 14A and 14B illustrate a model configuration and a coilconfiguration determined therefrom, respectively, for a head coil, inaccordance with some embodiments;

FIGS. 15A and 15B illustrate a model configuration and a coilconfiguration determined therefrom, respectively, for a leg coil, inaccordance with some embodiments;

FIGS. 16A and 16B illustrate coil configurations applied to a substratefor a head coil and a leg coil, respectively, in accordance with someembodiments;

FIG. 17 illustrates a conductor applied to a substrate in accordancewith a coil configuration, in accordance with some embodiments;

FIGS. 18A and 18B illustrate coil configurations for a head coil havingprinciple axes substantially orthogonal to one another, in accordancewith some embodiments;

FIG. 18C illustrates the combined coil configurations illustrated inFIGS. 18A and 18B, in accordance with some embodiments;

FIGS. 19A and 19B illustrate coil configurations for a leg coil havingprinciple axes substantially orthogonal to one another, in accordancewith some embodiments;

FIG. 19C illustrates the combined coil configurations illustrated inFIGS. 19A and 19B, in accordance with some embodiments;

FIG. 20 illustrates combined coil configurations for a head coil havingprincipal axes substantially parallel to one another, in accordance withsome embodiments;

FIG. 21 illustrates exemplary coil configurations for a head coilapplied to separate substrate layers of a support structure, inaccordance with some embodiments;

FIGS. 22A and 22B illustrate views of separate substrate layers of asupport structure to which exemplary coil configurations for a head coilare applied, in accordance with some embodiments;

FIGS. 23A and 23B illustrate views of a head coil having conductorsarranged according to respective coil configurations having principalaxes substantially orthogonal to one another;

FIG. 24 illustrates exemplary coil configurations for a leg coil appliedto separate substrate layers of a support structure, in accordance withsome embodiments;

FIG. 25 illustrates a leg coil having conductors arranged according torespective coil configurations having principal axes substantiallyorthogonal to one another;

FIGS. 26A and 26B illustrate a controller configured to operate amultifunction coil, in accordance with some embodiments; and

FIG. 27 illustrates a controller configured to operate a multifunctioncoil using a particular geometry for a gradient coil, in accordance withsome embodiments.

DETAILED DESCRIPTION

The MRI scanner market is overwhelmingly dominated by high-fieldsystems, and particularly for medical or clinical MRI applications. Asdiscussed above, the general trend in medical imaging has been toproduce MRI scanners with increasingly greater field strengths, with thevast majority of clinical MRI scanners operating at 1.5 T or 3 T, withhigher field strengths of 7 T and 9 T used in research settings. As usedherein, “high-field” refers generally to MRI systems presently in use ina clinical setting and, more particularly, to MRI systems operating witha main magnetic field (i.e., a B0 field) at or above 1.5 T, thoughclinical systems operating between 0.5 T and 1.5 T are often alsocharacterized as “high-field.” By contrast, “low-field” refers generallyto MRI systems operating with a B0 field of less than or equal toapproximately 0.2 T, though systems having a B0 field of between 0.2 Tand approximately 0.3 T have sometimes been characterized as low-fieldas field strengths have increased in the high-field regime.

Low-field MRI has been explored in limited contexts for non-imagingresearch purposes and narrow and specific contrast-enhanced imagingapplications, but is conventionally regarded as being unsuitable forproducing clinically useful images, particularly at field strengthssubstantially below 0.2 T (e.g., 100 mT or less). For example, theresolution, contrast, and/or image acquisition time is generally notregarded as being suitable for clinical purposes such as, but notlimited to, tissue differentiation, blood flow or perfusion imaging,diffusion-weighted (DW) or diffusion tensor (DT) imaging, functional MRI(fMRI), etc. The inventors have developed techniques for producingimproved quality, portable and/or lower-cost low-field MRI systems thatcan improve the wide-scale deployability of MRI technology in a varietyof environments beyond the large MRI installments at hospitals andresearch facilities.

A challenge in low-field MRI is the relatively low signal-to-noiseratio. In particular, the signal-to-noise ratio of an MR signal isrelated to the strength of the main magnetic field B0, and is one of thefactors driving clinical systems to operate in the high-field regime.Thus, the MR signal strength is relatively weak in the low-field contextdue to the low field strengths, increasing the importance of being ableto detect as much signal as possible. Some aspects of the inventors'contribution derive from their recognition that performance of alow-field MRI system may be improved by optimizing the configuration ofradio frequency (RF) transmit and/or receive coils (referred to hereinas RF transmit/receive coils or simply RF coils) to improve the abilityof the RF transmit/receive coils to transmit magnetic fields and detectemitted MR signals. As discussed above, low-field MRI systems produceweaker MR signals than their high-field counterparts, making it moreimportant that RF transmit/receive coils operate optimally (e.g., byboth transmitting optimal magnetic pulses and detecting as much of theemitted MR signals with as much fidelity as possible) in view of thelower signal-to-noise ratio (SNR).

Briefly, MRI involves placing a subject to be imaged (e.g., all or aportion of a patient) in a static, homogenous magnetic field B0 to aligna subject's atomic net magnetization (often represented by a netmagnetization vector) in the direction of the B0 field. One or moretransmit coils are then used to generate a pulsed magnetic field B1having a frequency related to the rate of precession of atomic spins ofthe atoms in the magnetic field B0 to cause the net magnetization of theatoms to develop a component in a direction transverse to the directionof the B0 field. After the B1 field is turned off, the transversecomponent of the net magnetization vector precesses, its magnitudedecaying over time until the net magnetization re-aligns with thedirection of the B0 field if allowed to do so. This process produces MRsignals that can be detected, for example, by electrical signals inducedin one or more receive coils of the MRI system that are tuned toresonate at the frequency of the MR signals.

MR signals are rotating magnetic fields, often referred to as circularlypolarized magnetic fields that can be viewed as comprising linearlypolarized components along orthogonal axes. That is, an MR signal iscomposed of a first sinusoidal component that oscillates along a firstaxis and a second sinusoidal component that oscillates along a secondaxis orthogonal to the first axis. The first sinusoidal component andthe second sinusoidal component oscillate 90° out-of-phase with eachother. An appropriately arranged coil tuned to the resonant frequency ofthe MR signals can detect a linearly polarized component along one ofthe orthogonal axes. In particular, an electrical response may beinduced in a tuned receive coil by the linearly polarized component ofan MR signal that is oriented along an axis approximately orthogonal tothe current loop of the coil, referred to herein as the principal axisof the coil.

Accordingly, MRI is performed by exciting and detecting emitted MRsignals using transmit/receive coils (also referred to interchangeablyas radio-frequency (RF) coils or Tx/Rx coils), which may includeseparate coils for transmitting and receiving, multiple coils fortransmitting and/or receiving, or the same coils for transmitting andreceiving. To transmit excitation pulse sequences and to detect emittedMR signals, transmit/receive coils must resonate at a frequencydependent on the strength of the B0 field. Accordingly, transmit/receivecoils in the high-field regime must resonate at significantly higherfrequencies (shorter wavelengths) than their low-field counterparts. Thelength of a conducting path of a resonant coil is constrained by thefrequency at which the resonant coil is intended to resonate. Inparticular, the higher the frequency, the shorter the conductive pathmust be for the resonant coil to operate satisfactorily. Thus, theconducting paths of high-field transmit/receive coils are required to bevery short. To meet this requirement, high-field transmit/receive coilsare frequently single turn conductive loops formed by etching, cuttingor milling conductive sheets (e.g., copper sheets). Typical conductingpaths for high-field transmit/receive coils are limited in length totens of centimeters.

The low frequencies involved in low-field MRI permit the conductingpaths of transmit/receive coils to be quite long, allowing for coildesigns that are not suitable (or useable) for high-field MRI due to theconstraints on conductive path length imposed by the high frequenciesinvolved in high-field MRI. According to some embodiments, atransmit/receive coil may be formed using a single conducting pathprovided over a three-dimensional surface corresponding to a region ofinterest. Due, in part, to relaxation of the constraint on conductorlength, the conducting path of the transmit/receive may be arranged overthe three-dimensional surface in a plurality of turns or loops. As usedherein, a “turn” refers to a conductive path provided 360° orsubstantially 360° about a reference axis (e.g., the principal axis ofthe coil, as discussed in further detail below). It should beappreciated that a turn need not form a closed loop provided theconductive path is formed substantially 360° about the reference axis.For example, a conductor arranged in a spiral geometry may comprisemultiple turns, though each turn does not form a closed loop. Exemplarycoils having conductors arranged in a plurality of turns are discussedin further detail below. By providing a coil having multiple turns(e.g., 5, 10, 15, 20, 30, 50 turns or more), the sensitivity of the coilin responding to MR signals can be improved.

The increase in allowable conductor length also allows for coils havinga single conductor arranged to cover an arbitrary geometry to facilitatetransmit/receive coils configured for desired portions of the anatomy.To image the head, for example, a low-field transmit/receive head coilmay be produced by winding a conductor about a substrate manufactured tobe worn by a person as a helmet. The conductor may be arranged, forexample, by positioning (e.g., winding) the conductor in a spiralgeometry about the surface of the helmet to provide coverage sufficientto provide transmit pulses to a region of interest (e.g., the brain or aportion thereof) and/or to detect MR signals emitted from the region ofinterest. As another example, to image the torso or an appendage (e.g.,a leg or a portion thereof, such as the knee), a conductor may besimilarly arranged in a spiral geometry about a surface configured toaccommodate the desired anatomy.

The above describe transmit/receive coil geometry is made possible byaspects of the low-field regime. As discussed above, the low fieldstrengths allow for significantly longer conductive paths to beutilized. In addition, clinical high-field MRI systems typicallygenerate a B0 field via a solenoid coil wound about a cylindrical boreinto which the patient being imaged is inserted. As such, the B0 fieldis oriented along the longitudinal axis of the bore and the bodyinserted therein. To perform MRI, transmit/receive coils produce a B1field perpendicular to the B0 field and detect emitted MR signals inthis transverse direction. This places restrictions on the geometry fortransmit/receive coils designed for high-field MRI. Low-field MRIfacilitates the design of “open” systems in which the B0 field isgenerated using, for example, bi-planar magnets between which a patientbeing imaged is placed such that the B0 field is substantially orientedperpendicular to the longitudinal axis of the body. For example, any ofthe low-field systems described in U.S. application Ser. No. 14/845,652('652 Application), titled “Low-field Magnetic Resonance Imaging Methodsand Apparatus,” and filed Sep. 4, 2015, or U.S. application Ser. No.14/846,255 ('255 Application), titled “Ferromagnetic Augmentation forMagnetic Resonance Imaging,” and filed Sep. 4, 2015, each of which isherein incorporated by reference in its entirety.

Accordingly, transmit/receive coils are arranged to produce and/ordetect magnetic fields transverse to this B0 field, allowing forgeometries not possible in traditional high-field MRI systems. As aresult, B0 magnets configured in arrangements that produce a B0 fieldthat is transverse to the axis of the body (e.g., bi-planar B0 magnets)allow for the design of transmit/receive coils that produce/detectmagnetic fields in the axial direction of the body, some examples ofwhich are described in further detail below. Transmit/receive coilsconfigured to respond to MR signal components oriented substantiallyalong the longitudinal axis of the body or specific target anatomy(i.e., coils configured with a principal axis substantially aligned withthe longitudinal axis of the body) are generally not useable with B0coils that produce magnetic fields aligned with the axis of the body,such as those commonly used in high-field MRI. However, it should beappreciated that transmit/receive coils may also be configured toperform MR signal detection in conjunction with MRI systems having a B0magnet that produces a B0 field in a direction aligned with thelongitudinal axis of the body (e.g., B0 magnet having a solenoidgeometry). In particular, according to some embodiments, an RF coil isprovided having a conductor with a plurality of turns configured torespond to MR signal components oriented orthogonal to the longitudinalaxis of the body, some examples of which are described in further detailbelow.

The inventors have appreciated that one or more of the different factorsregarding transmit/receive coils in the high-field and low-field contextfacilitate optimizing the design for transmit/receive coils for use inlow-field MRI. To this end, the inventors have developed techniques foroptimizing the configuration of RF coils to improve the performance ofthe coils for use with a low-field MRI system.

The inventors have appreciated that factors arising from the low-fieldcontext facilitate the use of magnetic field synthesis techniques toproduce generally optimal coil designs for RF coils. Magnetic fieldsynthesis is a technique for modeling coil(s) and simulating themagnetic fields generated by the modeled coil(s) when energized.Parameters of the coil models may then be adjusted to find a set ofparameters that generate a desired magnetic field according to somecriteria given one or more constraints on the coil models and/orparameters of the coil models. Due to several factors, magnetic fieldsynthesis techniques were heretofore generally inapplicable to designingRF coils for high-field MRI systems. In particular, such magnetic fieldsynthesis techniques were not effective in designing RF coils for usewith high-field MRI systems due in part to the relatively highfrequencies at which such coils are required to resonate when used inthe high-field regime. Specifically, the higher the frequency ofoperation, the shorter the current paths required to transmit andreceive. As a result, known magnetic field synthesis techniques were notuseful in designing receive coils with the short current paths needed,for example, to detect MR signals in the high-field context. Forexample, magnetic synthesis techniques may not be useful and/or orneeded to configure a single turn conductor with a short current pathtypically used in high-field MRI.

As discussed above, the significantly lower operation frequencies fortransmit and receive in low-field MRI (i.e., the significantly lowerfrequencies of transmit pulses and of emitted MR signals) allow forsubstantially longer current paths than for high-field MRI, which hasled to innovative new designs for RF coils for use in low-field MRIsystems. For example, a general rule of thumb is that the length of theconductor in a resonant coil should not exceed one tenth of thewavelength at the resonant frequency. Thus, a high-field MRI system witha B0 magnetic field of 3 T operates at approximately 128 MHz and so hasa wavelength of approximately 2.3 meters. Thus, the length of theconductors in the transmit/receive coils for such a high-field systemshould not exceed 23 centimeters. By contrast, a low-field MRI systemwith a B0 field of 0.1 T operates at approximately 4.3 MHz and so has awavelength of approximately 70 meters and therefore transmit/receivecoils can include conductors having lengths up to approximately 7meters. A low-field MRI system with a B0 field of 0.05 T operates atapproximately 2.15 MHz (−140 meter wavelength) and correspondingtransmit/receive coils can utilize conductors having lengths up to 14meters, and so on. The inventors have recognized that the significantlylonger conductor lengths permitted in the low-field regime allow fortransmit/receive coil configurations not possible in the high-fieldregime. In addition, the increased conductor lengths facilitate the useof magnetic field synthesis to determine optimal transmit/receive coilconfigurations.

The inventors have recognized that magnetic field synthesis techniquesmay be used to design RF coils for low-field MRI, and have developedtechniques to optimize the configuration of RF coil(s) to improvetransmission efficiency and/or improve efficacy in detecting MR signalsemitted in a low-field MRI environment. The inventors have developed RFcoil configurations that increase the sensitivity of MR signaldetection, thus improving the SNR of the system.

As discussed above, MR signals are rotating or circularly polarizedmagnetic fields. The inventors have developed RF coil designs configuredfor the low-field regime comprising a plurality of coils havingrespective different principal axes to respond to differently orientedmagnetic field components of MR signals (herein referred to as MR signalcomponents) to improve the SNR of MR signal detection. For example, afirst coil and a second coil may be arranged to have respectiveprincipal axes that are orthogonal or substantially orthogonal to oneanother (i.e., quadrature coils) to respond to orthogonal components ofemitted MR signals (e.g., to detect orthogonal linearly polarizedcomponents of a circularly polarized MR signal). In this manner, thepair of coils obtain dual measurements of an MR signal shifted in phaseby 90°, which measurements may be used to improve the SNR of MR signaldetection by, for example, combining the dual measurements, as discussedin further detail below.

It should be appreciated that the respective principal axes of aplurality of coils may be oriented with respect to each other in otherrelationships (e.g., non-orthogonal relationships). For example,orthogonality of the principal axes of a pair of coils for a givensurface may be difficult to achieve. In general, the improvement in SNRincreases the closer the principal axes of a pair of coils are toorthogonal, up to an improvement of the square root of two.Additionally, coils for which the principal axes are not orthogonal mayexhibit mutual inductance and may require configuring the respectivecoils in a manner to mitigate mutual inductance, some techniques ofwhich are described in further detail below.

According to some embodiments, an RF transmit/receive componentconfigured to respond to MR signals comprises a first coil formed by atleast one conductor arranged in a plurality of turns or loops accordingto a first coil configuration having a first principal axis and a secondcoil formed by at least one conductor arranged in a plurality of turnsor loops according to a second coil configuration having a secondprincipal axis different from the first principal axis. For example, thefirst coil configuration and the second coil configuration may be suchthat the first principal axis and the second principal axis aresubstantially orthogonal to one another, although other relationshipsbetween the respective principal axes may be used as well. In thismanner, the first and second coils can detect different components of anMR signal (e.g., orthogonal linearly polarized components of acircularly polarized MR signal) to improve the SNR of MR signaldetection. According to some embodiments, the first and second coilconfigurations for the first and second coils, respectively, aredetermined using magnetic synthesis techniques, though the coilconfigurations may be determined using other techniques (e.g., humanintuition, empirically, etc.), as the aspects are not limited in thisrespect. According to some embodiments, the first coil and the secondcoil are arranged on separate layers of a support structure to providean RF transmit/receive component having improved SNR, some examples ofwhich are described in further detail below.

The inventors have further appreciated that an optimal configuration fora receive coil may differ from one individual to another. For example,the size and shape of an individual's head may impact the optimalconfiguration for an RF coil for that individual. To address thisvariability, the inventors have developed techniques for optimizing oneor more receive coils for a specific individual. According to someembodiments, measurements of a target anatomy (e.g., head measurements,torso measurements, appendage measurements, etc.) of the specificindividual are obtained and the optimization techniques described hereinare performed using the obtained measurements. As a result, an optimalconfiguration for receive coil(s) may be obtained for a specificindividual. According to some embodiments, a support for the receivecoils for the target anatomy (e.g., a helmet) is fabricated (e.g., viathree-dimensional (3D) printing) in accordance with the optimalconfiguration determined. As a result, an optimal RF coil can be quicklyand cost effectively produced and may be customized for a particularindividual and/or portion of the anatomy.

The techniques described herein enable radio frequency components havingimproved sensitivity to MR signals, thus increasing the signal-to-noiseratio of MR signal detection. As discussed above, relatively weak MRsignals are a challenge of low-field MRI. Thus, transmit/receivecomponents produced using one or more techniques described hereinfacilitate low-field MRI systems capable of acquiring clinically usefulimages (e.g., images having resolutions suitable for clinical purposes,for example, diagnostic, therapeutic and/or research purposes). In thisrespect, some embodiments include a low-field MRI system comprising aradio frequency coil having at least one conductor arranged in a threedimensional geometry about a region of interest in a configurationoptimized to increase sensitivity to MR signals emitted within theregion of interest. For example, the low-field MRI system may comprise aB0 magnet configured to produce a low-field strength (e.g., between 0.2T and 0.1 T, between 0.1 T and 50 mT, between 50 mT and 20 mT, between20 mT and 10 mT, etc.) B0 magnetic field having a field of view, whereinthe radio frequency coil is optimized to provide radio frequency pulsesto the field of view to cause an MR response and/or to detect MR signalsemitted therefrom with improved efficacy.

Some embodiments include a dual coil radio frequency component having apair of coils configured for the low-field regime and oriented torespond to different MR signal components to improve the signal-to-noiseratio of MR signal detection. For example, some embodiments include alow-field magnetic resonance system comprising a B0 magnet configured toproduce a low-field strength (e.g., between 0.2 T and 0.1 T, between 0.1T and 50 mT, between 50 mT and 20 mT, between 20 mT and 10 mT, etc.) B0magnetic field having a field of view suitable for imaging, a first coilconfigured to be responsive to first MR signal components emitted fromthe field of view, and a second coil configured to be responsive tosecond MR signal components emitted from the field of view. In thisrespect, to respond to MR signals emitted from the field of view of thelow-field strength B0 magnetic field, the first coil and the second coilare configured to detect MR signals at frequencies corresponding to theB0 magnetic field (i.e., in the low-field regime).

According to some embodiments, the first coil and second coil arearranged to respond to orthogonal MR signal components (e.g., theprincipal axes of the first coil and the second coil are substantiallyorthogonal to one another) to maximize the boost in SNR, although otherarrangements can be used as well. According to some embodiments, thefirst coil and the second coil are offset from one another relative tothe field of view. According to some embodiments, the respectiveconfigurations of the first coil and the second coil are optimized, forexample, using magnetic synthesis techniques, though the respectiveconfigurations may be determined using other techniques (e.g.,intuition, empirically, etc.).

According to some embodiments, the B0 magnet of a low-field MRI systemis arranged in a planar geometry (e.g., a single-sided or a bi-planargeometry) and in other embodiments the B0 magnet is arranged in acylindrical geometry (e.g., a solenoid geometry), and the one or moreradio frequency coils are configured to transmit radio frequency pulsesand/or detect MR signals in accordance with the geometry of the B0magnet.

Following below are more detailed descriptions of various conceptsrelated to, and embodiments of, methods and apparatus for producing RFcoils, for example, for use in low-field MRI. It should be appreciatedthat the embodiments described herein may be implemented in any ofnumerous ways. Examples of specific implementations are provided belowfor illustrative purposes only. It should be appreciated that theembodiments and the features/capabilities provided may be usedindividually, all together, or in any combination of two or more, asaspects of the technology described herein are not limited in thisrespect.

FIG. 1 is a block diagram of exemplary components of a MRI system 100(e.g., a low-field MRI system). In the illustrative example of FIG. 1 ,MRI system 100 comprises computing device 104, controller 106, pulsesequences store 108, power management system 110, and magneticscomponents 120. It should be appreciated that system 100 is illustrativeand that a MRI system may have one or more other components of anysuitable type in addition to or instead of the components illustrated inFIG. 1 .

As illustrated in FIG. 1 , magnetics components 120 comprise B₀ magnet122, shim coils 124, RF transmit and receive coils 126, and gradientcoils 128. Magnet 122 may be used to generate the main magnetic fieldB₀. Magnet 122 may be any suitable type or combination of magneticscomponents that can generate a desired main magnetic B₀ field (e.g., anyone or combination of electromagnet(s), printed magnetics, permanentmagnet(s), etc.). Thus, a B₀ magnet refers herein to any one orcombination of magnetics components of any type configured to produce aB₀ field. According to some embodiments, B₀ magnet 122 may produce orcontribute to a B0 field greater than or equal to approximately 20 mTand less than or equal to approximately 50 mT, greater than or equal toapproximately 50 mT and less than or equal to approximately 0.1 T,greater than or equal to approximately 0.1 T and less than or equal toapproximately 0.2 T, greater than or equal to approximately 0.2 T andless than or equal to approximately 0.3 T, greater than 0.3 T and lessthan or equal to approximately 0.5 T, etc. Shim coils 124 may be used tocontribute magnetic field(s) to improve the homogeneity of the B₀ fieldgenerated by magnet 122.

Gradient coils 128 may be arranged to provide gradient fields and, forexample, may be arranged to generate gradients in the B0 field in threesubstantially orthogonal directions (X, Y, Z). Gradient coils 128 may beconfigured to encode emitted MR signals by systematically varying the B0field (the B0 field generated by magnet 122 and/or shim coils 124) toencode the spatial location of received MR signals as a function offrequency or phase. For example, gradient coils 128 may be configured tovary frequency or phase as a linear function of spatial location along aparticular direction, although more complex spatial encoding profilesmay also be provided by using nonlinear gradient coils. For example, afirst gradient coil may be configured to selectively vary the B0 fieldin a first (X) direction to perform frequency encoding in thatdirection, a second gradient coil may be configured to selectively varythe B0 field in a second (Y) direction substantially orthogonal to thefirst direction to perform phase encoding, and a third gradient coil maybe configured to selectively vary the B0 field in a third (Z) directionsubstantially orthogonal to the first and second directions to enableslice selection for volumetric imaging applications.

As discussed above, MRI is performed by exciting and detecting emittedMR signals using transmit and receive coils, respectively (oftenreferred to as radio frequency (RF) coils). Transmit/receive coils mayinclude separate coils for transmitting and receiving, multiple coilsfor transmitting and/or receiving, or the same coils for transmittingand receiving. Thus, a transmit/receive component may include one ormore coils for transmitting, one or more coils for receiving and/or oneor more coils for transmitting and receiving. Transmit/receive coils arealso often referred to as Tx/Rx or Tx/Rx coils to generically refer tothe various configurations for the transmit and receive magneticscomponent of an MRI system. These terms are used interchangeably herein.In FIG. 1 , RF transmit and receive coils 126 comprise one or moretransmit coils that may be used to generate RF pulses to induce anoscillating magnetic field B1. The transmit coil(s) may be configured togenerate any suitable types of RF pulses. For example, the transmitcoil(s) may be configured to generate any of the pulse sequencesdescribed in U.S. patent application Ser. No. 14/938,430 ('430application), titled “Pulse Sequences for Low Field Magnetic Resonance,”filed Nov. 11, 2015, which is herein incorporated by reference in itsentirety.

Each of magnetics components 120 may be constructed in any suitable way.For example, in some embodiments, one or more (e.g., all) of magneticscomponents 120 may be fabricated, constructed or manufactured usingtechniques described in U.S. patent application Ser. No. 14/845,652('652 application), titled “Low-field Magnetic Resonance Imaging Methodsand Apparatus,” and filed Sep. 4, 2015, which is herein incorporated byreference in its entirety. However, the techniques described herein arenot limited in this respect, as any suitable technique may be used toprovide the magnetics components 120.

Power management system 110 includes electronics to provide operatingpower to one or more components of the low-field MRI system 100. Forexample, as discussed in more detail below, power management system 110may include one or more power supplies, gradient power components,transmit coil components, and/or any other suitable power electronicsneeded to provide suitable operating power to energize and operatecomponents of the low-field MRI system 100.

As illustrated in FIG. 1 , power management system 110 comprises powersupply 112, power component(s) 114, transmit/receive switch 116, andthermal management components 118. Power supply 112 includes electronicsto provide operating power to magnetic components 120 of the MRI system100. For example, power supply 112 may include electronics to provideoperating power to one or more B₀ coils (e.g., B₀ magnet 122) to producethe main magnetic field for the low-field MRI system. In someembodiments, power supply 112 is a unipolar, continuous wave (CW) powersupply, however, any suitable power supply may be used. Transmit/receiveswitch 116 may be used to select whether RF transmit coils or RF receivecoils are being operated.

Power component(s) 114 may include one or more RF receive (Rx)pre-amplifiers that amplify MR signals detected by one or more RFreceive coils (e.g., coils 126), one or more RF transmit (Tx) powercomponents configured to provide power to one or more RF transmit coils(e.g., coils 126), one or more gradient power components configured toprovide power to one or more gradient coils (e.g., gradient coils 128),and one or more shim power components configured to provide power to oneor more shim coils (e.g., shim coils 124).

Thermal management components 118 provide cooling for components oflow-field MRI system 100 and may be configured to do so by facilitatingthe transfer of thermal energy generated by one or more components ofthe low-field MRI system 100 away from those components. Thermalmanagement components 118 may include, without limitation, components toperform water-based or air-based cooling, which may be integrated withor arranged in close proximity to MRI components that generate heatincluding, but not limited to, B₀ coils, gradient coils, shim coils,and/or transmit/receive coils. Thermal management components 118 mayinclude any suitable heat transfer medium including, but not limited to,air and liquid coolant (e.g., water), to transfer heat away fromcomponents of the low-field MRI system 100.

As illustrated in FIG. 1 , MRI system 100 includes controller 106 (alsoreferred to as a console) having control electronics to sendinstructions to and receive information from power management system110. Controller 106 may be configured to implement one or more pulsesequences, which are used to determine the instructions sent to powermanagement system 110 to operate the magnetic components 120 in adesired sequence. For example, for embodiments wherein MRI system 100operates at low-fields, controller 106 may be configured to controlpower management system 110 to operate the magnetic components 120 inaccordance with a zero echo time (ZTE) pulse sequence, a balancedsteady-state free precession pulse sequence (bSSFP), a gradient echopulse sequence, a spin echo pulse sequence, an inversion recovery pulsesequence, arterial spin labeling, diffusion weighted imaging (DWI),and/or any other pulse sequence specified for operation in the low-fieldcontext. Pulse sequences for low-field MRI may be applied for differentcontrast types such as T1-weighted and T2-weighted imaging,diffusion-weighted imaging, arterial spin labeling (perfusion imaging),Overhauser imaging, etc. However, any pulse sequence may be used, as theaspects are not limited in this respect. Controller 106 may beimplemented as hardware, software, or any suitable combination ofhardware and software, as aspects of the disclosure provided herein arenot limited in this respect.

In some embodiments, controller 106 may be configured to implement apulse sequence by obtaining information about the pulse sequence frompulse sequences repository 108, which stores information for each of oneor more pulse sequences. Information stored by pulse sequencesrepository 108 for a particular pulse sequence may be any suitableinformation that allows controller 106 to implement the particular pulsesequence. For example, information stored in pulse sequences repository108 for a pulse sequence may include one or more parameters foroperating magnetics components 120 in accordance with the pulse sequence(e.g., parameters for operating the RF transmit and receive coils 126,parameters for operating gradient coils 128, etc.), one or moreparameters for operating power management system 110 in accordance withthe pulse sequence, one or more programs comprising instructions that,when executed by controller 106, cause controller 106 to control system100 to operate in accordance with the pulse sequence, and/or any othersuitable information. Information stored in pulse sequences repository108 may be stored on one or more non-transitory storage media.

As illustrated in FIG. 1 , controller 106 also interacts with computingdevice 104 programmed to process received MR data. For example,computing device 104 may process received MR data to generate one ormore MR images using any suitable image reconstruction process(es).Controller 106 may provide information about one or more pulse sequencesto computing device 104 for the processing of data by the computingdevice. For example, controller 106 may provide information about one ormore pulse sequences to computing device 104 and the computing devicemay perform an image reconstruction process based, at least in part, onthe provided information.

Computing device 104 may be any electronic device that may processacquired MR data and generate one or more images of the subject beingimaged. In some embodiments, computing device 104 may be a fixedelectronic device such as a desktop computer, a server, a rack-mountedcomputer, a workstation, or any other suitable fixed electronic devicethat may be configured to process MR data and generate one or moreimages of the subject being imaged. Alternatively, computing device 104may be a portable device such as a smart phone, a personal digitalassistant, a laptop computer, a tablet computer, or any other portabledevice that may be configured to process MR data and generate one orimages of the subject being imaged. In some embodiments, computingdevice 104 may comprise multiple computing devices of any suitable type,as aspects of the disclosure provided herein are not limited in thisrespect. A user 102 may interact with computing device 104 to controlaspects of the low-field MR system 100 (e.g., program the system 100 tooperate in accordance with a particular pulse sequence, adjust one ormore parameters of the system 100, etc.) and/or view images obtained bythe low-field MRI system 100.

FIGS. 2A and 2B illustrate exemplary bi-planar geometries for a B0magnet. B0 magnet 222 is schematically illustrated by magnet 222 a and222 b arranged substantially parallel to one another to generate a B0field generally along axis 245 in whichever (direction is desired toprovide a field of view between the magnets 222 a and 222 b (i.e., aregion between the magnets wherein the homogeneity of the B0 field issuitable for MRI). This bi-planar arrangement allows for the productionof a generally “open” magnetic resonance imaging system. An RF coil (ormultiple RF coils) is schematically illustrated as RF coil 226, whichis/are arranged to generate a pulsed oscillating magnetic fieldgenerally along axis 225 (i.e., the principal axis of RF coil 226) tostimulate an MR response and to detect MR signals. Exemplary RF coil 226is arranged to detect the MR signal component oriented substantiallyalong the principal axis 225 (i.e., linearly polarized components of theMR signal aligned with the coil's principal axis). As discussed above,the relatively low operational frequencies of low-field MRI allow forcoil designs that are not suitable for use in the high-field context.The inventors have developed RF coil designs that improve the ability ofthe coils to transmit RF pulse sequences and/or to detect emitted MRsignals, some of which are discussed in further detail below. Theinventors have further developed techniques for optimizing thearrangement of conductor(s) for an RF coil according to a desiredcriteria using magnetic synthesis, some examples of which are alsodescribed in further detail below.

FIGS. 3A and 3B illustrate several views of a radio-frequency (RF) headcoil configured to transmit appropriate RF pulse sequences in alow-field MRI system and to detect the emitted MR signals responsive tothe RF pulse sequence. Transmit/receive coil 300 may, for example,correspond to RF coil 226 illustrated in FIG. 2 and configured inparticular to obtain MR images of the head. As shown, transmit/receivecoil 300 includes a substrate 350 formed to accommodate the head of asubject to be imaged. The substrate may be formed with grooves in whichconductor 330 is provided (e.g., wound) according to a desiredgeometry). The substrate includes a helmet portion to accommodate thehead and a support base so that a patient can comfortably rest the headwithin the helmet in a resting position.

As illustrated, conductor 330 is wound about substrate 350 in a spiralgeometry so that, when operated, the coil produces a magnetic field indirections along axis 305, and can detect magnetic fields oriented alongthe same axis. As such, axis 305 corresponds to the principal axis ofthe coil formed by conductor 330. Conductor 330 comprises a singlecontinuous wire forming a single channel transmit and receive coil. Theexemplary transmit/receive coil 300 in FIGS. 3A and 3B has a conductingpath of approximately 14 meters. As discussed above, the highfrequencies of high-field MRI (e.g., greater than 64 Mhz) requireconducting paths of RF coils to be very short to operate correctly(e.g., on the order of centimeters). Thus, the length of the conductorin this exemplary transmit/receive coil is well beyond (by an order ofmagnitude or more) the limit imposed by the high frequencies of thehigh-field MRI regime. However, the illustrated configuration is notoptimized, and as a result, the performance of the head coil may besub-optimal resulting in lower quality images.

The inventors have developed RF coil configurations to improve coilefficacy (e.g., improve RF pulses delivered to a subject and/or improvethe sensitivity in detecting MR signals emitted in response to RF pulsesequences). As a result, increased signal can be detected resulting inhigher SNR, which is a particularly important factor in low-field MRIwhere MR signals are relatively weak compared to high-fieldcounterparts. The inventors have further developed techniques todetermine a generally optimal arrangement (e.g., configuration) ofconductors on RF coils to improve the ability of the resulting coil(s)to detect emitted MR signals in the low-field context and/or to transmitRF energy. As discussed in further detail below, the techniquesdescribed herein can be applied to any surface of interest to provide RFcoils having any desired geometry for any portion or portions of theanatomy (e.g., head, torso, arms, legs, knees, etc.).

FIG. 4A illustrates a method of determining an RF coil configuration, inaccordance with some embodiments. In act 410, a model of the RF coil isprovided. The term “model” refers herein to any mathematicalrepresentation of an RF coil or representation from which arepresentation of an RF coil can be derived. For example, a model of anRF coil may include a geometric representation such as a triangulatedmesh or other representation built from geometric primitives.Additionally, the model may be described by implicit surfaces and/or mayinclude other types of suitable mathematical representations orcombinations thereof. Suitable models generally allow for magnetic fieldsynthesis to be performed using the model, for example, by allowing theoperation of the modeled RF coil to be simulated to synthesize themagnetic fields generated within a region of interest upon simulatedoperation. A model typically has one or more parameters that, when setto a given set of respective values, characterizes a particularconfiguration of the model. Varying the values of one or more of theparameters varies the configuration of the model. An optimized RF coilconfiguration can be derived from an optimized model configuration, forexample, by finding a configuration of the model (e.g., the set of oneor more parameters describing the model) that is optimal according to agiven criteria, as discussed in further detail below.

In act 420A, a configuration for the RF coil is determined using themodel of the RF coil. For example, an optimization may be performedusing the model to determine a configuration for the RF coil thatsatisfies at least one constraint and that, when operation of the modelis simulated, produces a magnetic field satisfying at least onecriteria. According to some embodiments, the at least one criteria forthe magnetic field includes magnetic field homogeneity. For example, anoptimization may be formulated such that it identifies a configurationfor the model that produces a magnetic field within a region of interestthat meets a homogeneity criterion (e.g., non-uniformity of less than aspecified percentage) when the model of the RF coil is simulated.According to some embodiments, the at least one criteria includes amagnetic field strength criterion. Any suitable criterion or combinationof criteria may be used that facilitates determining a desiredconfiguration for the RF coil from an optimized configuration of themodel. According to some embodiments, the model configuration and the RFcoil configuration are described using different parameters. Forexample, the model configuration may represent a surface potentialhaving parameters corresponding to current densities, and the RF coilconfiguration represent the arrangement of a conductor (e.g., wires) inthree dimensional space. According to some embodiments, an optimal modelconfiguration may be identified (e.g., by determining an optimal set ofparameters according to a given criteria) and an RF coil configurationmay be determined from the optimized model configuration. Determiningthe RF coil configuration may involve a second optimization, but inother embodiments, the RF coil configuration is determined in other ways(e.g., determining an optimal coil configuration may involve multiplestages). According to some embodiments, the optimal RF coilconfiguration is determined in conjunction with optimizing the modelconfiguration. For example, the model configuration and the RF coilconfiguration may be similarly parameterized such that the optimal RFcoil configuration is generally determined by optimizing the modelconfiguration, depending on how the RF coil is modeled.

As discussed above, an optimization may include finding optimalparameter values that satisfy a given criteria in view of at least oneconstraint. The at least one constraint may be any constraint orcombination of constraints that facilitates a configuration (either amodel and/or coil configuration) that meets one or more designspecifications for the RF coil. According to some embodiments, the atleast one constraint includes resistance of the RF coil configuration.For example, the optimization may enforce a maximum resistance for an RFcoil configuration or otherwise minimize coil resistance in determiningan optimal configuration with respect to a given criteria. According tosome embodiments, the at least one constraint includes inductance of theRF coil. For example, the optimization may enforce a maximum inductancefor an RF coil configuration or otherwise minimize coil inductance indetermining an optimal configuration with respect to a given criteria.Any other constraint or combination of constraints may additionally oralternatively be used to determine a coil configuration, some examplesof which are described in further detail below.

As a result of performing step 420A, a configuration of the RF coil isobtained. As discussed above, a coil configuration may be defined by aset of parameters describing the RF coil. According to some embodiments,the configuration of the RF coil describes the three dimensionalgeometry of one or more conductors (e.g., describes how one or moreconductors are arranged in three-dimensional space). For example, theconfiguration may describe the number of turns or loops and the spacingbetween turns of at least one conductor of the RF coil and/or any otherdescription of how the at least one conductor is arranged. Theconfiguration may be any description of how one or more conductors of anRF coil are arranged over a surface of interest and/or any descriptionof characteristics and/or properties of the one or more conductors, asthe aspects are not limited in this respect. According to someembodiments, a coil configuration is determined from a modelconfiguration obtained by optimizing one or more parameters of an RFcoil model. For example, the RF coil configuration may includeparameters that govern the number of turns of the RF coil, the spacingbetween turns and/or the location of the conductor (e.g., wire(s)) inthe coil), etc. In general, the one or more parameters of the RF coilconfiguration define, at least in part, the distribution and/orarrangement of the physical conductors over the surface of interest ofthe physical RF coil. Further details in connection with some examplesof coil optimization techniques are discussed below.

FIG. 4B illustrates a method of optimizing a configuration for an RFcoil, in accordance with some embodiments. In act 410B, a model of theRF coil is obtained. The model may be obtained or provided using any ofthe techniques discussed above in connection with FIG. 2A, or using anysuitable technique for providing a representation of the RF coil.

In act 420B, operation of the modeled RF coil is simulated for aparticular configuration of the model. For example, given a particularmodel configuration, the magnetic fields generated by simulatingoperation of the model are synthesized. According to some embodiments,simulation may involve synthesizing the magnetic fields generated at aset of points within a region of interest by simulating currents on thesurface of the model of the RF coil. In act 422, the synthesizedmagnetic fields are compared to a given criteria to evaluate whether theconfiguration is satisfactory from an optimization perspective (e.g.,whether it satisfies a given criteria). According to some embodiments,the criteria may take the form of a function having one or moreconstraints and/or one or more variables to be minimized and/ormaximized. For example, the optimization of the function may seek tomaximize the magnetic field generated within a region of interest whileminimizing the inductance and/or resistance of the RF coil (orconstraining the inductance and/or resistance to be below respectiveprescribed values). However, any set of variables in view of any set ofconstraints may be used, as the techniques described herein are notlimited for use with any particular optimization or optimization scheme.

The particular coil design and design constraints may dictate, at leastin part, what factors are considered in optimizing the configuration ofan RF coil. Non-limiting factors that may be evaluated in anoptimization formulation for the design of an RF coil (e.g., in the formof variables to be minimized or maximized, or as constraints) includeany one or combination of magnetic field strength, magnetic fieldhomogeneity, coil efficiency/sensitivity, coil inductance, coilresistance, wire length, wire thickness, wire spacing, etc. The relativeimportance of any one or combination of these factors may be weighted sothat an optimal configuration according to given design constraints maybe obtained.

If it is determined in act 422 that the solution (e.g., the evaluationof a given function) resulting from simulating the operation of themodel with the current configuration is optimal according to apredetermined measure, the process proceeds to act 460, where an RF coilconfiguration is determined based on the model configuration. Forexample, the optimized model configuration may be used to determine acoil configuration that, when operated, will produce a magnetic fieldapproximately like the magnetic field simulated from the modelconfiguration. According to some embodiments, the coil configuration isdetermined from the optimized model configuration by determining wirecontours for an RF coil based, at least in part, on the modelconfiguration. For example, a contouring technique, an example of whichis discussed below, may be used to determine wire contours for theoptimized RF coil, and the wire contours can subsequently be used togenerate the actual physical RF coil as illustrated in act 470, furtheraspects of which are described below. That is, the contours describe acoil configuration and may be used as the pattern with which to arrangethe physical conductors of the RF coil.

If it is determined in act 422 that the solution is not optimalaccording to the predetermined measure (e.g., does not satisfy a givencriteria), the process proceeds to act 430, where one or more parametersof the model may be modified to produce an updated model configuration.In optimizing the model configuration, the process returns to act 420 tosimulate the operation of the RF coil using the updated modelconfiguration, and the process iterates until the optimum configurationis determined (e.g., the set of one or more parameters governing themodel configuration are optimized according to a given criteria). Themanner in which the configuration is updated for the next iteration canbe chosen in accordance with any suitable optimization scheme. Byrepeating acts 420, 422, and 430, the configuration of the model of theRF coil may be optimized according to some measure characterized by thecriteria (e.g., by optimization of a suitable function). From the finalmodel configuration, a generally optimal RF coil configuration may beobtained. It should be appreciated that optimizing a model configurationand/or an RF coil configuration need not result in a global or absoluteoptimal solution, but need only converge to some sufficient measure of“optimal.” As such, for a given model and formulation, there may benumerous “optimal” solutions.

FIG. 5 illustrates an example implementation of the general methoddescribed in FIG. 4 , in accordance with some embodiments. In act 510, amodel of an RF coil is provided using a three-dimensional mesh of asurface corresponding to a region of interest to which RF energy is tobe provided and MR signals are to be detected (e.g., the field of viewof a low-field MRI system). According to some embodiments, the meshcomprises a plurality of surface elements connected by nodes at thevertices of the surface elements. Non-limiting examples of a mesheshaving triangular surface elements that may be used as a basis for amodel of an RF coil in accordance with some embodiments are shown inFIGS. 6A and 6B. In particular, FIG. 6A illustrates an exemplary mesh600A corresponding to a head coil. Mesh 600A is formed by a plurality oftriangles (e.g., triangle 610) connected by sharing sides with one ormore adjacent triangles. Each triangle vertex or node (e.g., node 620)is shared by one or more adjacent triangles, although any suitableconfiguration of surface elements may be used to form a mesh. In someembodiments, the mesh comprises approximately 1000-4000 triangles,though it should be appreciated that any suitable number of trianglesmay be used, and the number and/or shape of triangles in the mesh maydepend, at least in part, on the surface being modeled.

FIG. 6B illustrates an exemplary mesh 600B corresponding to an RF coiladapted for imaging the leg, for example, the knee or other portionthereof. Like the mesh in FIG. 6A, the desired surface is triangulatedto form a plurality of triangles (e.g., triangle 610) interconnected atshared vertices or nodes (e.g., node 620). It should be appreciated thatthat the exemplary surfaces illustrated in FIGS. 6A and 6B are merelyillustrative and a mesh can be defined for any arbitrary geometry usingany desired primitive. That is, surface elements having any geometricshape (e.g., triangles, squares, hexagons, octagons, etc.) may be usedto define a mesh over any surface. It should be further appreciated thatusing a mesh is merely one example of a geometric representation thatmay be suitable for use in providing a model of an RF coil.

A mesh, such as those illustrated in FIGS. 6A and 6B, provides aflexible representation to model an RF coil, as any arbitrary surfacecan be represented using a mesh, thus facilitating the modeling of an RFcoil for any desired portion of the anatomy of the human body,including, but not limited to the head, neck, torso, one or moreappendages or portions thereof (e.g., arms, legs, hands, feet orportions thereof), and/or any combination of anatomical portions toproduce an RF coil optimized for use with any desired portion of thehuman body.

Referring again to FIG. 5 , in act 520, operation of the model of the RFcoil may be simulated. For example, using the example triangular meshes600A or 600B illustrated in FIGS. 6A and 6B, operation of the model maybe simulated, at least in part, by simulating current loops about eachnode in the mesh (e.g., by simulating current loops through adjacenttriangles around their shared nodes) and computing the magnetic fieldgenerated by the respective current loops at designated target pointsselected within a region of interest. Specifically, a number of targetpoints (e.g., 100-1000 designated points on the interior of thetriangular mesh) may be selected at which to compute the magnetic fieldsresulting from simulating current loops about the nodes of thetriangular mesh. Generally speaking, the target points are selected anddistributed in a manner so as to suitably characterize the magneticfields throughout the region of interest. The region of interest may,for example, be associated with the field of view of the imaging system,but may correspond to other regions of interest as well.

According to some embodiments, current loops are simulated at each nodein the mesh and the resulting magnetic field generated by each of thecurrent loops at each target point is determined to obtain informationregarding the effect of each current loop on each of the target points.For example, simulating operation of the model in this manner can beused to obtain a matrix of data corresponding to the magnetic fieldgenerated at each of the target points in response to each respectivecurrent loop that is simulated. This data can in turn be operated on bya suitable optimization algorithm, examples of which are described infurther detail below. According to some embodiments, the strength ofeach of the current loops forms, at least in part, a set of parametersthat are varied during optimization. That is, a suitable optimizationalgorithm selects the strength for each of the current loops to, forexample, maximize or minimize a given function (e.g., a potentialfunction defined on the surface of the mesh by the current loops) orother suitably formulated optimization to achieve desired magnetic fieldcharacteristics at each of the target points in the region of interest.

Following simulation of the operation of the RF coil using surfacecurrent loops, the example process of FIG. 5 proceeds in a similarmanner as discussed above in connection with the example process of FIG.4 . For example, according to some embodiments, operation of the modelmay be performed by simulating current loops around the nodes of thesurface elements (e.g., vertices of the triangular mesh) to optimize apotential function defined on the surface of the mesh. In such anexample optimization, iterating through acts 520, 522 and 530 shown inFIG. 5 results in an optimized surface potential achieved, at least inpart, by varying the strengths of the current loops simulated over thetriangulated mesh until the magnetic field generated satisfies a givencriteria. FIGS. 7A and 7B illustrate model configurations 705 a and 705b, respectively. In the exemplary embodiments illustrated in FIGS. 7Aand 7B, the model configuration is characterized, in part, by surfacepotentials that have been optimized using techniques described herein.In particular, the shading in FIGS. 7A and 7B depicts the magneticscalar surface potential (e.g., the stream function of the currentdensity, as discussed in further detail below in connection with theexemplary optimization illustrated in FIG. 12 ), the value of which aredetermined during the optimization. From this surface potentialfunction, a coil configuration may be determined, as discussed infurther detail below. In the exemplary embodiments illustrated in FIGS.7A and 7B, the surface potential function corresponds to the integratedcurrent densities over the surface, obtained, at least in part, byvarying the current strength parameter of the current loops at the nodesin the mesh until simulation of the model configuration has beenoptimized to satisfy a given criteria in view of one or moreconstraints.

In act 560, an RF coil configuration may be determined from the modelconfiguration. For example, the model configuration 705 a and 705 b(e.g., a potential function) illustrated in FIGS. 7A and 7B may beconverted into contours indicating the arrangement of the conductors foran RF head coil and leg coil, respectively. FIGS. 8A and 8B illustrate acoil configuration 815 characterized by contour lines (e.g., exemplarycontour lines 880) for the conductor(s) of a head coil determined fromthe model configuration 705 a illustrated in FIG. 7A. For example, forthe exemplary coil configurations illustrated in FIGS. 8A and 8B, thecontour lines are selected so as to produce the current densities (i.e.,the differential of the surface potential function illustrated in FIGS.7A and 7B) of the optimized model configuration. Because the contourlines of a coil configuration represent the current paths of a coil thatmay ultimately be realized by a single conductor (e.g., a singleconductor wound to form a plurality of turns or loops in accordance withthe contours of the coil configuration), each contour line has the samecurrent. Thus, to achieve the varying current densities described by themodel configuration, the spacing of the contour lines are variedaccordingly. Specifically, regions of higher current density willproduce contours that are spaced closer together while regions of lowercurrent density will produce contours that are spaced further apart.Thus, a coil configuration may be determined from a model configurationby finding contour lines of equal potential over the surface potentialfunction of the model configuration (e.g., contour lines that passthrough equal scalar values of the surface potential functionsillustrated in FIGS. 7A and 7B). Determining a coil configuration from amodel configuration in this manner may be achieve, at least in part,using any suitable contouring or level set algorithm.

FIG. 8A illustrates coil configuration 815 overlaid on the modelconfiguration from which the contours were determined, and FIG. 8Billustrates coil configuration 815 by itself. The contour lines for theconductor of the RF coil are selected to produce substantially themagnetic field generated when simulating the model using the optimizedmodel configuration. In this manner, a generally optimal coilconfiguration 815 may be determined. That is, the exemplary coilconfiguration 815, characterized by the arrangement of the contours inspace, defines a conductor pattern optimized according to a desiredcriteria. As illustrated, the contours of the coil configuration 815have a principal axis 825, substantially aligned with the longitudinalaxis of the body. Principal axis 825 is also an exemplary reference axisabout which the coil configuration forms a plurality of turns.

As illustrated in FIGS. 8A and 8B, in the resulting RF coilconfiguration, the spacing between the contours (e.g., the spacingbetween turns in the conductor of the RF coil are non-uniform withcontours being more closely spaced towards the base of the RF coilconfiguration. Thus, in contrast to the coil illustrated in FIG. 3having a configuration based on human intuition that has substantiallyuniform spacing between the turns of the conductor of the coil acrossthe helmet surface, the optimized coil configuration illustrated inFIGS. 8A and 8B has non-uniform spacing between numerous contours suchthat the resulting RF coil will have non-uniform spacing betweennumerous turns or loops of the conductor forming the RF coil, aconfiguration that provides an optimal solution that is unlikely to bearrived at using human intuition alone or by empirical trial and error.Additionally, while the coil configuration in FIG. 3 has substantiallyregular contours, the optimized coil configuration results in multipleirregular contours. Thus, the optimization produces a configurationsolution unlikely to be arrived at when relying on human intuitionalone. FIGS. 9A and 9B illustrate an optimized RF leg coil configuration915 determined from the model configuration 705 b illustrated in 7B, thecoil configuration forming a plurality of turns about a principal axis925, which is substantially aligned with the longitudinal axis of thetarget anatomy (e.g., the patient's leg) when the target anatomy ispositioned within the coil.

An RF coil configuration (e.g., the exemplary coil configurationsillustrated in FIGS. 8A, 8B, 9A and 9B) may then be used to produce anRF coil in accordance with the determined configuration. For example, toproduce an RF coil the RF coil configuration typically will need to betransferred to a support structure, for example, a helmet to be worn bya subject for the head coil configuration 815 illustrated in FIGS. 8Aand 8B. According to some embodiments, an RF coil configuration is usedto produce an RF coil by applying contours of the RF coil configurationto a substrate, which is in turn used to fix the arrangement of theconductor over the surface of the RF coil. FIGS. 10A and 10B illustratedifferent views of a geometrical rendering of a helmet 1000 with grooves(e.g., grooves 1080) formed in a substrate 1050 corresponding to thelocations computed for the conductor during the exemplary optimizationdescribed in the foregoing. For example, grooves 1080 may be provided incorrespondence to the contours of coil configuration 815 determined frommodel configuration 705 a. In particular, the contours of a coilconfiguration may be mapped to the surface of a support structure orsubstrate to provide the locations at which to apply the coil conductor(e.g., locations to provide grooves for the coil conductor). Thedimensions of the grooves (e.g., the width and depth of the grooves) maybe chosen so as to accommodate the conductor to be used to form theradio frequency coil. This surface, once rendered (e.g., from thesurface mesh and optimized coil configuration), can then be manufactured(e.g., using a 3D printer) to quickly and cost effectively produce ahelmet for an RF head coil, for example, for use in low-field MRI. Asillustrated in FIG. 10A, a groove 1085 is provided to connect thegrooves 1080 corresponding to the contours for the conductor (e.g.,corresponding to the contours of the coil configuration obtained from anoptimized model configuration). Groove 1085 allows a single conductor tobe wound about substrate 1150 within the provided grooves to provide aplurality of turns of the conductor, as discussed in further detailbelow. When a conductor is positioned within the grooves, the conductorwill form a plurality of turns about principle axis 1025, as illustratedin FIG. 10A. When a patient's head is positioned within helmet 1000, theprinciple axis may be directed in substantial alignment with thelongitudinal axis of the patient's body.

FIG. 11 illustrates an exemplary substrate or support 1100 to which theRF coil configuration 915 in FIGS. 9A and 9B has been applied to producethe support for a leg coil. In particular, support 1100 comprisesgrooves (e.g., grooves 1180), formed in substrate 1150, corresponding tothe contours of the exemplary RF coil configuration 915 illustrated inFIGS. 9A and 9B. Support 1100 includes groove 1185 provided to connectgrooves 1180 to facilitate positioning a conductor continuously withingrooves 1180 in a plurality of turns in accordance with a desired coilconfiguration. When a conductor is positioned within the grooves, theconductor will form a plurality of turns about principle axis 1125.

Once a support structure is produced (e.g., helmet 1000, support 1100 orother geometry configured for particular anatomy), the conductor (e.g.,wire) can be applied to the structure (e.g., by positioning theconductor within the grooves) to produce an RF coil with an optimizedcoil configuration. For example, a single conductor may be positionedwithin grooves formed in a support structure manufactured in accordancewith the geometry of the respective RF coil (e.g., a wire may be placedwithin grooves 1080, 1180 illustrated in FIGS. 10 and 11 , respectively)to produce, at least in part, an RF coil with improved transmit/receiveproperties. It should be appreciated that a coil configuration can beapplied to a support structure using an suitable technique and is notlimited to providing grooves in the substrate support structure. Thatis, a conductor may be coupled to a support structure according to adesired coil configuration in any suitable manner, as the aspects arenot limited in this respect. An RF coil produced using an optimized coilconfiguration may exhibit increased sensitivity to emitted MR signals,improving the SNR of a low-field MRI system. Further examples of RFcoils manufactured using techniques described herein are discussed indetail below.

The ease of which such support structures can be manufacturedfacilitates producing custom RF coils for particular individuals and/orparticular parts of the body. In connection with customizing RF coilsfor particular individuals, measurements of the particular individualmay be obtained using lasers or other range finding equipment and/or viamanual measurements using, for example, calipers to take measurements ofimportant dimensions of the portion of anatomy being imaged. Themeasurements and/or range data may be used to create a surface for usein modeling an RF coil (e.g., the measurement data may be used to rendera mesh corresponding to the geometry of the anatomy of interest for thespecific patient). Optimization techniques described herein may then beperformed to locate an optimal RF coil configuration, which in turn canbe used to produce (e.g., via 3D printing) a support for the optimalcoil configuration that is customized for the particular patient. As aresult, optimized coil configurations can be determined and thecorresponding coil produced relatively quickly and efficiently for anyarbitrary geometry of interest.

As discussed above, the design of an RF coil may involve meeting certaindesign constraints and/or requirements. According to some embodiments,coil inductance and/or coil resistance are evaluated to constrain theoptimization of the RF coil configuration. As discussed above, tooperate correctly, RF transmit/receive coils are resonated. Thus, anincrease in inductance requires an increase in capacitance in the tuningcircuit coupled to the coil to achieve resonance. Increased resistanceimpacts the quality (Q) factor of the coil by increasing the bandwidthof the coil's resonance, rendering the coil less effective instimulating an MR effect and less sensitive in detecting emitted MRsignals. A particular system may have design requirements specifyinginductance and/or resistance for the coil (e.g., to achieve a coilhaving a specified Q factor or to match a specified tuning circuit,etc.). Thus, by evaluating coil inductance and/or coil resistance (e.g.,by minimizing or setting limits on their values) an RF coilconfiguration can be optimized given the specified design constraints.

According to some embodiments, a regularization scheme is utilized thatincludes additional terms corresponding to one or more designconstraints (e.g., coil resistance, coil inductance, field homogeneity,etc.). For example, coil inductance and/or coil resistance may beincluded as additional terms in the optimization. In connection with theexample RF coil model illustrated in FIG. 6 , coil resistance and/orinductance may be computed for each of the simulated current loops. As aresult, data corresponding to magnetic field strength and one or moreadditional constraints such as coil resistance or inductance may begenerated. For example, a magnetic field strength matrix may be computedas a first term and a coil resistance matrix may be computed as a secondterm, wherein the optimization operates to achieve desired magneticfield characteristics while minimizing coil resistance. It should beappreciated that additional terms for any desired constraint may beincluded in the optimization. The selected terms can be weighted asdesired so that the optimization produces desired values (e.g., thevalues for the function on the surface of the mesh that produce anoptimal result in view of the specified constraints).

It should be appreciated that any number or types of constraints may beincluded in the optimization to meet the requirements of a particulardesign. For example, a given design may require the use of a wire ofgiven thickness or width. To prevent the optimization from selecting aconfiguration where wires are positioned too close together (e.g., asolution where the spacing between wires (e.g., turns of the conductor)at one or more locations on the surface is less than the width of thewire), a term may be included in the optimization that imposes a minimumspacing between wire forming the conducting path(s) of the coil. Coilresistance constraints may be implemented by including a term in theoptimization corresponding to wire length for designs using wireconductors with fixed thickness, as discussed in further detail below.

An example implementation of a method for determining a configuration ofan RF coil is described in further detail below in connection with theillustrative and non-limiting process illustrated in FIG. 12 . It shouldbe appreciated that the below described implementation is merely oneexample of how to optimize an RF coil configuration and that any othersuitable techniques may be used, as determining a configuration for anRF coil using a model of the RF coil is not limited to any particularimplementation. In act 1210, a surface geometry to be modeled isreceived. As discussed above, any arbitrary surface geometry may be usedfor generating an RF coil in accordance with the techniques describedherein. In act 1212, a model of the surface geometry is created. In thisexemplary model, the surface geometry may be considered as a thinconducting surface S, defined at a point r′ by the unit normal vector tothe surface; {circumflex over (n)}(r′). The current flowing on S isrepresented at r′ by the current density vector J(r′). When the currentdensity is constrained to the surface S and is divergence free, apotential function, the stream-function may be defined over the surfaceS. The current density on the surface S generates magnetic field B(r)over a region of interest V separated from the surface S. Therelationship between the generated magnetic field B(r) and the currentdensity on the surface S may be stated as:

$\begin{matrix}{{{dB}(r)} = {\frac{\mu_{0}{Id}1 \times \left( {r^{\prime} - r} \right)}{4\pi r^{3}}.}} & (1)\end{matrix}$

Optimization may be performed, at least on part, by solving the inverseproblem to find the current density J(r′) on the surface S that willprovide a given magnetic field B(r) over the region of interest V. Tosolve this inverse problem, the problem may be discretized. For thesurface S, the current density J (r′) may be discretized using a meshdefined by a set of flat triangular surface elements with nodes at thecorners of the surface elements (e.g., as illustrated in FIG. 6 ). Asdiscussed above, shapes of surface elements other than triangles mayalternatively be used to form the mesh used to discretize the surface S.A stream function ψ(r′) of the current density may be discretized as aset of basis functions for each node I_(n) of the mesh as:

$\begin{matrix}{{\psi\left( r^{\prime} \right)} \approx {\sum\limits_{n = 1}^{N}{I_{n}{{\psi_{n}\left( r^{\prime} \right)}.}}}} & (2)\end{matrix}$

In (2), ψ_(n)(r′) is the stream-function basis-function for the nth nodeof the mesh. The above example stream function for a node describes acurrent loop flowing on the surface S through all triangle elements ofthe mesh that share the node. Nodes on the edge of the mesh may beforced to have the same stream function value to prevent current fromflowing in and out of the edge. In the inverse solution, the streamfunction values at each node of the mesh act as free parameters that canbe optimized using the techniques described herein.

The process then proceeds to act 1214, where the magnetic field B(r) inthe region of interest V is discretized. The magnetic field may bediscretized by defining a set of target points that reside within theregion V. The target points may have any position in space other than onthe surface S, and together define the target region of interest V. Insome embodiments, described in more detail below, the set of targetpoints may include first target points corresponding to a first regionin which a maximum magnetic field is desired and second target pointscorresponding to a second region in which a minimum (e.g., zero)magnetic field is desired. For example, the first target points may belocated in a volume inside of the surface S, whereas the second targetpoints may be located outside of the surface S. The inclusion of thesecond target points enables the design of RF coils providing shieldingbenefits in addition to optimizing the coil design to provide a desiredmagnetic field in a region to be imaged, for example, the field of viewof a low-field MRI system.

The process of FIG. 12 then proceeds to act 1216, where the modelconfiguration is optimized by, for example, determining the optimumvalues for the current density on the surface S as modeled by the streamfunction at each node of the mesh for the desired magnetic field at theset of target points. In addition to the desired magnetic field, someembodiments also include other parameters desired to be minimized duringoptimization, such as the stored energy (inductance) in the coil orresistive power dissipation in the coil. Boundary conditions may also beimposed as constraints during optimization. For example, to conservecurrent over the surface S, the condition that the potential be the samefor all points along an edge of the surface may be imposed via one ormore constraints. For example, to conserve current of the surface of thehead coil illustrated in FIG. 6A, a condition that the potential be thesame for points along the single edge may be enforced as a constraint inthe optimization. Similarly, points along the edges on either end of theleg coil surface illustrated in FIG. 6B may also be constrained to haveequal potential to other points along the same edge, though thepotentials along the two edges are allowed to be different. It should beappreciated that a surface may be formed from any number of separatesurface, each of which may have any number of edges. An exemplaryfunction U to be minimized using a suitable optimization scheme may beexpressed as follows:

$\begin{matrix}{U = {{\frac{1}{2}{\sum\limits_{k = 1}^{K}\left( {{B\left( r_{k} \right)} - {B^{t}\left( r_{k} \right)}} \right)^{2}}} + {\frac{\alpha}{2}{\sum\limits_{n = 1}^{N}{\sum\limits_{m = 1}^{N}{I_{n}I_{m}L_{mn}}}}} + {\frac{\beta}{2}{\sum\limits_{n = 1}^{N}{\sum\limits_{m = 1}^{N}{I_{n}I_{m}{R_{mn}.}}}}}}} & (3)\end{matrix}$

In (3), the first term describes the difference between the measuredfield and the target field, the second term models the inductanceL_(mn), and the third term models the coil resistance R_(mn). Theinductance and resistance terms can be weighted using regularizationterms α and β determined based on desired features of the RF coil beingdesigned. In some embodiments, the minimum of the function U may beidentified by differentiating the function with respect to I_(n) toproduce a linear system of equations that can be consolidated into amatrix equation: ZI=b, where the matrix Z is calculated by thedifferentiating optimization and the vector b contains the magneticfield values. This matrix equation may then be inverted to solve for I,which contains the stream function values I_(n) at each of the nodes nof the mesh. The nodal stream function values I_(n) can then be linearlycombined to reconstruct the stream function of the current density overthe surface S. Thus, the above described optimization of a surfacepotential function may be used to determine an optimized modelconfiguration, for example, the optimized model configurations 705 a and705 b illustrated in FIGS. 7A and 7B, respectively. However, it shouldbe appreciated that the above described method is merely exemplary, andany function and constraints may be optimized to obtain an optimizedmodel configuration and will depend on the nature and characteristics ofthe model and the requirements of the design.

According to some embodiments, additional constraints may be added tothe optimization problem including, but not limited to, requiring aminimum spacing of wires (e.g., between adjacent turns of theconductor(s)) and/or reducing the total length of the coil conductor(e.g., wire length). As another example, in the context of multi-channelreceive coils (e.g., for performing parallel MRI), a further constraintthat minimizes the mutual inductance between a given coil and anothercoil may be included in the optimization scheme (e.g., a constraint thatrequires or seeks to reduce the mutual inductance between pairs of coilsto zero or satisfactorily close to zero). Such a constraint facilitatesthe design of multiple receive coil arrays that are substantiallydecoupled from each other during receive operations.

Introduction of additional constraints into the optimization maycomplicate or compromise the ability to solve the matrix equation aboveusing a simple inversion technique. Accordingly, some embodimentsminimize the function U using a convex optimization technique ratherthan matrix inversion. For example, optimization of the coil design maybe achieved by using Tikhonov regularized minimization of theroot-mean-squared (RMS) residual to minimize ∥βψ−b_(t)∥∥₂+α∥ψ∥₂, whereb_(t) is the target field and a is a regularization parameter. Inembodiments using a convex optimization, any suitable convexoptimization solver may be used, as the aspects are not limited in thisrespect. It should be appreciated that other optimization techniques mayalso be suitable, including, but not limited to, gradient descent,genetic algorithms, particle swarm, simulated annealing, Monte Carlotechniques, etc.

Returning to the process of FIG. 12 , after an optimum solution for themodel configuration has been determined, the process may proceed to act1218, where the stream function for the current density output from act1216 is used to generate a coil configuration, for example, arepresentation of conductor contours that, when supplied with a current,produce the desired magnetic fields for the optimized coil design. Insome embodiments, a contouring technique is used to determine theposition of conductor(s) (e.g., wire(s)) on the surface S for theoptimized coil configuration. Contouring may be performed in anysuitable way. For example, each element (e.g., a triangle) of the meshused to approximate the surface S may be transformed into parametric(u,v) space by linear transformation. The values of the stream-functionat the corners of the element (e.g., for a triangle element (ψ₁, ψ₂,ψ₃)) may be used to define a plane of the stream-function in the elementin (u, v, ψ) space. The intersection of this plane with planes ofconstant ψ, representing the contour levels ψC_(n), gives the equationof the conductor paths in that element. The portion of these lines thatare within the u and v limit of the unit element are the wire paths ofthat element. The process may be carried out for all elements andtransformed back into (x,y,z) space, with the result being the conductorpaths of the coil configuration. In some embodiments, constraints areadded during contouring to constrain the solution based on one or morephysical properties, such as the width dimension of the conductor (e.g.,the cross-sectional diameter of the wire), as discussed above.

Once the conductor paths for the RF coil are known, the process of FIG.12 proceeds to act 1220, where a support structure for the RF coil isgenerated and the coil configuration applied to the support structure.In some embodiments, a three-dimensional (3D) printer or other suitabledevice may be used to generate the support structure for optimized RFcoil designs, as discussed above. The support structure may include oneor more channels, grooves or conduits corresponding to the location ofthe conductor paths resulting from determining the configuration of theRF coil (e.g., the conductor paths resulting from contouring theoptimized stream function values discussed above). That is, the coilconfiguration may be used to determine the location of groovesconfigured to accommodate the coil conductor in accordance with the coilconfiguration. The support structure may be provided in other ways tofacilitate applying one or more conductors according to the RF coilconfiguration determined via optimization. The process then proceeds toact 1222, where conductor(s) (e.g., a wire) is/are provided along thepaths on the support structure to create an RF coil based on theoptimized configuration. An appropriate resonant circuit may then becoupled to the coil to produce an RF coil optimally configured forperforming transmit and/or receive, for example, as part of a low-fieldMRI system. In particular, the coil may be tuned to resonate at a targetfrequency in the low-field regime.

As discussed above, in the low-field context, the relatively lowtransmit frequencies allow the length of the conductors to besubstantially increased with respect to the conductor lengths in thehigh-field regime. For example, the conductor path illustrated in theexemplary RF coil configuration applied to the support structureillustrated in FIGS. 10A and 10B is approximately 4 meters in length,which exceeds the maximum length restrictions in the high-field contextby an order of magnitude or more. According to some embodiments, theconductor length is greater than 1 meter, greater than 2 meters, greaterthan 4 meters, greater than 7 meters, greater than 10 meters, etc.Accordingly, transmit/receive coils that operate optimally according todesired criteria may be designed and produced relatively simply and costeffectively and may operate with relatively high efficiency.

In addition to the flexibility of design afforded by increased conductorlength, the substantial relaxation of this constraint allows the RF coilto be formed using a single conductor, wound in multiple turns, usingsingle strand wire of suitable gauge or multi-stranded wire such as aLitz wire. For example, the configuration illustrated in FIGS. 10A and10B comprises 20 turns or loops for the conductor. However, any numberof turns can be selected or determined via an optimization and maydepend on the geometry of the coil and desired operating characteristicsthereof. Generally speaking, increasing the number of turns or loops ofthe coil conductor increases the sensitivity of the coil. However, theinventors have recognized that at a certain point, increasing the numberof turns may in fact degrade performance of the RF coil. In particular,a coil comprising multiple turns or loops will resonate without beingtuned (self-resonate) at least partially due to a parasitic capacitancearising from the relationship of the conductor between the multipleturns or loops in the coil. The effect of the self-resonance is toreduce the Q-factor of the coil and degrading its performance. Thiseffect may be particularly deleterious when the self-resonanceapproaches the frequency at which the RF coil is tuned to resonate(i.e., the target resonant frequency of the coil corresponding to thestrength of the B0 field of the MRI system). Because the frequency ofthe self-resonance decreases as the number of turns increases, thisphenomena may place an effective limit on the number of turns of theconductor before the coil performance degrades unsatisfactorily.According to some embodiments, the number of turns of the conductor ofthe coil is limited to ensure that the frequency of the self-resonanceis at least twice that of the frequency of the target resonance to whichthe RF coil is tuned. According to some embodiments, the number of turnsof the conductor of the coil is limited to ensure that the frequency ofthe self-resonance is at least three times that of the frequency of thetarget resonance to which the RF coil is tuned, and according to otherembodiments, the number of turns of the conductor of the coil is limitedto ensure that the frequency of the self-resonance is at least fivetimes that of the target resonance.

The limit on the number of turns needed to ensure that the frequency ofthe self-resonance is a desired distance away from the frequency of thetarget resonance depends on a number of factors, including the geometryand size of the coil (e.g., the geometry of a head coil may result in adifferent limit than the geometry of a leg coil to achieve the sameseparation of the self-resonance and target resonance frequencies), andthe type of conductor being used (e.g., the gauge of the wire, whetherthe wire is single or multi-stranded, etc.). It should be appreciatedthat the limitation on the number of turns can be selected to any numberdepending on the requirements of the coil, including placing nolimitation on the number of turns of the conductor of the coil.

The inventors have developed transmit/receive coil configurations thatimprove the efficacy of the coil in transmitting RF pulses and/ordetecting MR signals emitted in response. As discussed above, theexemplary coils described in the foregoing are configured to detect thelinearly polarized components of MR signals oriented along the principalaxis of the coil (e.g., axis 1325 illustrated in FIG. 13A). However, thecircularly polarized MR signals emitted, for example, in theconfiguration illustrated in FIG. 13A, also include linearly polarizedcomponents oriented in an orthogonal direction illustrated by axis 1335(into and out of the plane of the drawing) that are not detected by theexemplary coils discussed in the foregoing. For example, as illustratedin FIGS. 13C and 13D, the exemplary head coil and the exemplary leg/kneecoil configurations described in the foregoing are configured to detectMR signal components oriented along axis 1325, but not MR signalcomponents oriented along axis 1335. FIG. 13B illustrates a B0 magnet1324 having a cylindrical geometry oriented in the same coordinate frameas the planar B0 magnet illustrated in FIG. 13A. For example, B0 magnetmay be a solenoid electromagnet that produces a B0 field along axis1325. As such, exemplary coil configurations illustrated in FIGS. 13Cand 13D are generally not useable in such a configuration because theprincipal axes of the coil configurations are aligned with the B0 field.The inventors have appreciated that RF coils can be configured to detectMR signal components oriented along axis 1335 and/or axis 1345 and suchconfigurations can, but need not, be optimized using the same techniquesdescribed in the foregoing. As such, RF coils can be configured todetect MR signals using any B0 magnet geometry (e.g., planar,cylindrical, solenoid, etc.) by configuring the RF coil to have aprincipal axis oriented appropriately relative to the direction of theB0 field.

By way of illustration, FIGS. 14A and 14B illustrate an exemplary modelconfiguration 1405 and RF coil configuration 1415 determined therefromadapted (e.g., optimized) to detect MR signal components oriented alongthe principal axis 1435 of a head coil, and FIGS. 15A and 15B illustratean exemplary model configuration 1505 and RF coil configuration 1515determined therefrom adapted (e.g., optimized) to detect MR signalcomponents along the principal axis 1535 of a leg/knee coil. Theprincipal axes 1435 and 1535 also correspond to an exemplary referenceaxis (it should be appreciated that there are multiple reference axes)about which the respective configuration form a plurality of turns. Asshown, the principle axes 1435 and 1535 are substantially orthogonal tothe longitudinal axis of the target anatomy when the target anatomy ispositioned within the respective coils.

As illustrated in FIGS. 14B and 15B, the principal axes 1435 and 1535are orthogonal to axes 1445 and 1545, respectively, along which a B0field may be oriented, for example, a B0 field generated by a bi-planarB0 magnet. As also illustrated in FIGS. 14B and 15B, the principal axes1435 and 1535 are orthogonal to axes 1425 and 1525, respectively, alongwhich a B0 field may be oriented, for example, a B0 field generated by asolenoid B0 magnet. Thus, the coil configurations 1415 and 1515 may beused to produce coils to transmit RF pulses and/or detect MR signals ina number of B0 magnet geometries. In a similar or same manner asdiscussed above, the exemplary RF coil configurations 1415 and 1515 maythen be applied to a support substrate by producing grooves or otherstructures to accommodate conductors of the coil (e.g., exemplary headcoil substrate 1650 a and leg coil substrate 1650 b illustrated in FIGS.16A and 16B, respectively) in accordance with the respective coilconfiguration and positioning a conductor (e.g., a wire) within thegrooves or otherwise affixing the conductor to the substrate in thearrangement described by the coil configuration (e.g., the contours ofthe exemplary coil configurations 1415 and 1515, respectively), thusforming a plurality of turns about exemplary reference axis 1635 a and1635 b, corresponding also to the principle axes of the respectivecoils.

FIG. 17 illustrates an exemplary head coil in which a conductor ispositioned within grooves formed in a support substrate in the form of ahelmet configured to accommodate a person's head to, for example,obtained one or more images of the patient's brain. In particular, headcoil 1700 includes a substrate 1750 having grooves or channels 1780arranged according to a desired coil configuration in which a conductor1725 is placed to form a plurality of turns or loops (e.g., illustrativeturns 1727) of the coil. Groove 1785 is provided to connect grooves 1780so that conductor 1725 can be wound about the support substrate from onecontour or loop to the next in accordance with a desired coilconfiguration. Exemplary head coil 1700 comprises 20 turns (10 turns oneach hemisphere) formed by the conductor loops of the coil configurationabout principal and exemplary reference axis 1735. As discussed above,the relatively long conductor lengths that can be used in the low fieldcontext allow a single conductor to be wound about the surface ofinterest in accordance with a desired coil configuration. It should beappreciated that according to some embodiments, a coil configuration isapplied using a plurality of conductors, which may be independent of oneanother or connected together. According to some embodiments, conductor1725 is formed from a suitable gauge wire. For example, conductor 1725may be a single stranded wire or may be a multi-stranded wire such as aLitz wire. It should be appreciated that conductor 1725 may be anysuitable conductor, as the aspects are not limited for use with anyparticular type of conductor.

As discussed in the foregoing, the inventors have recognized thatmultiple coil configurations can be used in conjunction to improve theSNR of an RF coil. For example, a pair of coils configured to havedifferent principal axes may be used to obtain dual measurements of MRsignals. According to some embodiments, an RF transmit/receive componentis provided comprising a first coil and a second coil configured to haverespectively orthogonal or substantially orthogonal principal axes toimprove the SNR of the RF component. For example, the exemplary headcoil configurations adapted to detect MR signal components orientedalong the principal axis 1325 of the exemplary coil configurationillustrated in FIGS. 13B) and the exemplary head coil configurationsadapted to detect MR signal components oriented along the principal axis1435 of the exemplary coil configuration illustrated in FIG. 14B),respectively, can be used together to detect MR signal componentsoriented along both principal axes. By utilizing such a dual coilarrangement, the SNR of MR signal detection may be improved, asdiscussed in further detail below.

By way of example, FIGS. 18A and 18B illustrate coil configurations thatcan be combined to provide a head coil capable of detecting MR signalcomponents oriented along multiple axes, in accordance with someembodiments. In particular, FIG. 18A illustrates an exemplary coilconfiguration 1815 a arranged to detect MR signal components orientedsubstantially along principal axis 1825 and FIG. 18B illustrates anexemplary coil configuration 1815 b arranged to detect MR signalcomponents oriented substantially along principal axis 1835 orthogonalto principal axis 1825. FIG. 18C illustrates a multiple coilconfiguration 1815 c, produced by combining coil configurations 1815 aand 1815 b, arranged to detect MR signal components orientedsubstantially along principal axes 1825 and 1835.

As a further example, FIGS. 19A and 19B illustrate exemplary coilconfigurations 1915 a and 1915 b configured to detect MR signalcomponents oriented along orthogonal principal axes 1925 and 1935,respectively, which may be combined to form coil configuration 1915 cillustrated in FIG. 19C to provide a multiple coil configurationarranged to detect MR signal components oriented along multipleorthogonal axes. By configuring multiple coils to detect MR signalcomponents oriented along substantially orthogonal axes, inductivecoupling between the coils can be optimally avoided. Using dual coilsconfigured with mutually orthogonal principal axes may, according tosome embodiments, boost the SNR of MR signal detection by the squareroot of two. In particular, each of the dual coils may obtain anindependent measurement of the same MR signal shifted in phase by 90°,resulting in a square root of two SNR improvement.

In the example coil configurations illustrated in FIGS. 18C and 19C, thedual coil configurations are oriented substantially orthogonal to oneanother and orthogonal to a B0 field. That is, the principal axes of thedual coils are orthogonal to one another and orthogonal to axis 1845,1945 along which the B0 field is aligned. However, the inventors haverecognized that other arrangements may also be used. For example, FIG.20 illustrates a combined coil configuration 2015 c for an exemplaryhead coil comprising coil configuration 2015 a having conductor(s)arranged to detect MR signal components generally oriented along axis2025 and coil configuration 2015 b having conductor(s) arranged todetect MR signal components generally oriented along axis 2035. In theexample configuration illustrated in FIG. 20 , axes 2025 and 2035 areorthogonal to one another and at 45° relative to axis 2045 a and 2045 b,in which directions possible B0 fields may be generated, for example, bya low-field MRI device. It should be appreciated that otherconfigurations are also possible, as the aspects are not limited in thisrespect. For example, a plurality of coils may be configured to detectMR signals in directions that are not orthogonal. However, in suchcases, care should be taken to produce coils configurations havingsuitably low mutual inductance. The inventors have recognized that theoptimization techniques described herein may be used to determine coilconfigurations in which the mutual inductance between the coils isminimized, as discussed in further detail below. In this way, aplurality of coils may be utilized that do not have orthogonalrelationships with one another.

To apply a plurality of coil configurations (e.g., the exemplary coilconfigurations 1815 c, 1915 c, 2015 c, etc.) to provide an RFtransmit/receive component comprising multiple coils (e.g., a pair ofquadrature coils), the inventors have appreciated that the conductor(s)forming the coil for respective configurations may be offset from oneanother. To separate a pair of coils arranged about a region ofinterest, the conductors of the coils may be offset from one anotherrelative to the region of interest. For example, the conductor of afirst coil may be arranged about the region of interest and theconductor of a second coil may be arranged about the region of interestat a distance further away from the region of interest. According tosome embodiments, a support structure comprises an inner substrate layerhaving a surface about a region of interest to which a first coil isapplied and an outer substrate layer having a surface about the regionof interest to which a second coil is applied. The inner substrate layerand the outer substrate layer may be, for example, offset from oneanother in directions normal to the substrate surfaces to which thecoils are applied. In this respect, the outer substrate layer isprovided over the inner substrate layer with respect to the region ofinterest. Some non-limiting examples of a dual coil radio frequencycomponent having a first coil provided in a first substrate layer of asupport structure and a second coil provided in a second substrate layerof the support structure offset from the first substrate layer aredescribed in further detail below. It should be appreciated, however,that multiple coils may be applied in other manners, as the aspects arenot limited in this respect.

By way of example of a dual coil radio frequency component, FIG. 21illustrates a helmet 2100 to which a pair of coil configurations areapplied to respective substrate layers of the helmet. In particular,coil configuration 2115 a (e.g., a coil configuration similar to or thesame as coil configuration 1815 a illustrated in FIG. 18A) is applied toan outer substrate layer 2155 a of the support structure of helmet 2100via grooves adapted to accommodate the coil conductor arranged inaccordance with the corresponding coil configuration. Substrate layer2155 a is illustrated in FIG. 21 with one of the hemispheres removed toillustrate the inner substrate layer underneath. In this respect, coilconfiguration 2115 b (e.g., a coil configuration similar to or the sameas coil configuration 1815 b illustrated in FIG. 18B) is applied to aninner substrate layer 2155 b of the support structure of helmet 2100 viagrooves adapted to accommodate the coil conductor arranged in accordancewith the corresponding coil configuration. As shown in FIG. 21 , thedirection of the offset of outer substrate layer 2155 a from innersubstrate layer 2155 b is normal to the substrate surfaces and, in thisillustrative example, outer substrate layer 2155 a overlays innersubstrate layer 2155 b.

As shown by exemplary helmet 2100, the inner and outer substrate layersform respective surfaces about a region of interest within the helmet.When the helmet is worn by a patient and operated within an appropriateB0 field of an MRI system, the region of interest will include the fieldof view of the MRI system (i.e., the region of the B0 field havingsufficient homogeneity to perform MRI). Thus, the exemplary substratelayers 2155 a and 2155 b illustrated in FIG. 21 are offset from oneanother relative to the region of interest, with outer substrate layer2155 a being arranged farther away from the region of interest thaninner substrate layer 2155 b. Consequently, when operated within asuitable B0 field of an MRI system, the coil applied to outer substratelayer 2155 a will be farther away from the field of view than the coilapplied to inner substrate layer 2155 b. When a conductor is positionedwithin the grooves of substrate layer 2155 a, the conductor forms aplurality of turns about principle axis 2125 (e.g., aligned with thelongitudinal axis of the body), and when a conductor is positionedwithin the grooves of substrate layer 2155 b, the conductor forms aplurality of turns about principle axis 2135 (e.g., substantiallyorthogonal to the longitudinal axis of the body).

When arranged in close proximity, coils provided in separate layers mayexhibit capacitive coupling. This capacitive coupling between coilsprovided in separate layers may be reduced or avoided by increasing thedistance between the coils in the different layers in the direction ofthe normal to the surface of the support structure. For example, byincreasing the offset of the coil in the outer layer in the direction ofthe surface normal, capacitive coupling can be reduced or eliminated.However, increased offsets also generally decrease the sensitivity ofthe coil in the outer layer due to the increased distance from theregion of interest, so the offset can be chosen to appropriately balancecapacitive coupling and coil sensitivity as appropriate and/or desired.Alternately, or in addition to, decoupling networks may be included toreduce or eliminate the capacitive coupling between coils provided inseparate layers.

In FIG. 21 , openings or slots 2175 may be provided to facilitateconnection of the hemispheres of outer layer 2155 a and/or toaccommodate the terminal ends of the coil conductor(s), once positionedwithin the grooves, to facilitate connection to the transmit and/orreceive circuitry that operates the RF head coil. In this manner,multiple coil configurations may be applied to a support structure toproduce an RF head coil having improved SNR.

FIGS. 22A and 22B illustrate an alternative technique for applyingmultiple coil configurations to a support structure for a helmet 2200.In FIG. 22A, coil configuration 2215 a (e.g., a coil configurationsimilar to or the same as coil configuration 1815 a illustrated in FIG.18A) is applied to an inner substrate layer 2255 a of the supportstructure of helmet 2200 via grooves adapted to accommodate the coil andposition the conductor in accordance with the corresponding coilconfiguration. FIG. 22B illustrates a hemisphere of an outer substratelayer 2255 b removed to show inner substrate layer 2255 a in FIG. 22Aand to illustrate coil configuration 2215 b applied to the insidesurface of outer substrate layer 2255 b. In particular, coilconfiguration 2215 b (e.g., a coil configuration similar to or the sameas coil configuration 1815 b illustrated in FIG. 18B) is applied to theinside of outer layer 2255 b (e.g., on the concave side of the outerlayer) via grooves adapted to accommodate and position the coilconductor in accordance with the corresponding coil configuration.Opening 2275 is configured to accommodate the conductor terminals forconnection to the transmit and/or receive circuitry and also may beadapted to attach the two portions of outer layer 2255 b. It should beappreciated that either coil configuration may be applied to the inneror outer layers in FIGS. 21 and 22 and the choice of the arrangementsshown are merely for illustration. In addition, it should be appreciatedthat coil configurations can be applied to either the concave or convexside of either the inner or outer substrate layers, and the arrangementillustrated is shown to illustrate that coil configurations can beapplied to either side of a substrate surface.

FIGS. 23A and 23B illustrate an RF head coil 2300 comprising a first RFcoil 2310 a formed by conductor 2327 a arranged according to a firstconfiguration (e.g., by positioning conductor 2327 a within groovespatterned in an inner layer according to configuration 2215 aillustrated in FIG. 22 ) and a second RF coil 2310 b formed by conductor2327 b arranged according to a second configuration (e.g., bypositioning conductor 2327 b within grooves patterned in an outer layeraccording to configuration 2215 b illustrated in FIG. 22 ), as shown inFIG. 23A. FIG. 23B shows the terminal end of conductors 2327 a and 2327b emerging from the opening in the support structure of head coil 2300for connection to the transmit and/or receive circuitry so that the RFhead coil can be operated, for example, to obtain one or more MRI images(e.g., one or more images of a patient's brain). For example, RF headcoil 2300 may be connected to a low-field MRI system to acquire MRsignals with improved SNR, thus improving the quality of the acquiredimages.

It should be appreciated that providing conductors for an RF coil byproviding grooves, channels or conduits according to a desiredconfiguration is merely one example of producing an RF coil that may besuitable, for example, when producing supports structures using 3Dprinting or similar techniques. However, any method or technique may beused to provide a conductor according to a desired configuration toproduce an RF coil. For example, one or more conductors may beencapsulated within support structure material in a molding process orother fabrication process, or one or more conductors may be affixed to asupport structure in other ways such as by fasteners, adhesives, etc.Any suitable technique for providing conductors in accordance with adesired configuration may be used, as the aspects are not limited inthis respect.

FIG. 24 illustrates a support structure 2400 for a leg coil to which apair of coil configurations are applied. In particular, coilconfiguration 2415 a (e.g., a coil configuration similar to or the sameas coil configuration 1915 a illustrated in FIG. 19A) is applied to anouter layer 2455 a of the support structure 2400 via grooves adapted toaccommodate and fix the coil conductor position in accordance with thecorresponding coil configuration. Coil configuration 2415 b (e.g., acoil configuration similar to or the same as coil configuration 1915 billustrated in FIG. 19B) is applied to an inner layer 2455 b of thesupport structure 2400 via grooves adapted to accommodate and fix thecoil conductor position in accordance with the corresponding coilconfiguration. Structure 2475 provides a mechanism to route the terminalends of the conductors, once positioned within the grooves of the coilconfigurations, for connection to the transmit and/or receive circuitryto operate the RF coil. In this manner, multiple coil configurations maybe applied to a support structure to produce an RF leg coil havingimproved SNR.

FIG. 25 illustrates an exemplary RF coil 2500 adapted for the legcomprising a first RF coil 2510 a formed by conductor 2527 a arrangedaccording to a first configuration (e.g., by positioning conductor 2527a in an outer layer according to configuration 1915 a illustrated inFIG. 19A) to form a plurality of turns about exemplary reference axis2525 (e.g., the principal axis substantially aligned with thelongitudinal axis of a leg positioned within the coil), and a second RFcoil 2510 b formed by conductor 2527 b arranged according to a secondconfiguration (e.g., by positioning conductor 2527 b in an inner layeraccording to configuration 1915 b illustrated in FIG. 19B) to form aplurality of turns about exemplary reference axis 2535 (e.g., theprincipal axis substantially orthogonal to the longitudinal axis of aleg positioned within the coil). RF coil 2500 may be used to obtain oneor more images of a portion of the leg, for example, one or more imagesof the knee as part of a low-field MRI system. Connector 2575 routes theterminal ends of conductors 2527 a and 2527 b and provides connectionsto electrically connect the conductors to the transmit and/or receivecircuitry of, for example, a low-field MRI system. It should beappreciated that the above described techniques may be used to produceRF coils for any portion of the anatomy and the exemplary head and legcoils depicted are merely examples to illustrate methods and apparatusdeveloped by the inventors and discussed herein.

In embodiments having a radio frequency component comprising multiplecoils, one or both of the coils may be used to transmit RF pulses to aregion of interest to cause an MR response. For example, in someembodiments, only one of a plurality of coils is used as a transmit coiland each of the plurality of coils is used as a receive coil. Accordingto some embodiments, each of the plurality of coils is used as atransmit coil and as a receive coil. As such, a plurality of coils maybe used in any arrangement to provide a transmit/receive component of amagnetic resonance imaging system, for example, a low-field MRI system.

In embodiments that include a plurality of coils (e.g., RFtransmit/receive components that utilize a pair of coils in a quadraturerelationship, as illustrated by exemplary RF coils 2300 and 2500illustrated in FIGS. 23 and 25 ), MR signals will produce electricalsignals in each of the plurality of coils. These signals may be combinedto improve SNR in any number of ways. For example, the electricalsignals may be combined in the analog or digital domain. In the analogdomain, electrical signals produced in each of the plurality of coilsmay be phase shifted appropriately and combined. For example, using theexemplary coils described above, the electrical signals produced in eachof the pair of coils from corresponding MR signals will be 90° out ofphase as a result of the orthogonality of the respective configurations.As such, electrical signals of one of the coils may be phase shifted by90° and combined with electrical signals produced by the other coil toobtain a combined signal having increased SNR. In the digital domain, MRsignals may be obtained over separate channels (e.g., separate signalsmay be obtained from each of the coils) and digitized. The digitizedsignals may then be processed digitally and combined in the digitaldomain by phase shifting the digitized signals. One advantage toobtaining separate signals and processing them in the digital domain isthe ability to perform noise correction on the individual signals beforecombining them. However, MR signal components detected by multiple coilsmay be combined and processed in any suitable way, as the aspects arenot limited in this respect.

As discussed above, the coil configurations of a radio frequencycomponent comprising multiple coils may be optimized using thetechniques described in the foregoing, for example, using magneticsynthesis to determine a coil configuration that is generally optimalwith respect to one or more parameters. According to some embodiments,mutual inductance between multiple coils may be included as a term in anoptimization scheme to minimize the mutual inductance between the coils.A mutual inductance term may be particularly beneficial in embodimentswhere the coil configuration are not oriented orthogonal to one another(e.g., coil configuration having principal axes that are not orthogonalto one another), either by design or because orthogonality cannot beachieved to the extent desired. Minimizing (or eliminating) mutualinductance between coils facilitates radio frequency components withimproved SNR and/or sensitivity, thus improving the quality of MR signaldetection.

A low-field MRI system may include a radio frequency component providedin accordance with any one or combination of the techniques described inthe foregoing to facilitate acquiring clinically useful images atlow-fields. For example, a low-field MRI system may include a B0 magnet122 configured to produce a low-field B0 magnetic field and atransmit/receive component 125 may be optimized to increase thesensitivity and/or configured to improve the SNR of MR signal detectionusing any one or combination of techniques described herein tofacilitate acquiring clinically useful images of desired portion(s) ofthe anatomy.

The inventors have further appreciated that a coil may be operated sothat the coil produces more than one type of magnetic field in an MRIsystem. For example, the inventors have developed systems that drive oneor more coils in a multifunction capacity to generate one or moregradient magnetic fields and to generate and/or receive one or more RFmagnetic fields. According to some embodiments, a multifunction coil isconfigured to operate as at least one transmit/receive coil and as atleast one gradient coil. The inventors have further recognized that theoptimization techniques described herein may be employed to optimize aconfiguration of such a multifunction coil. Further details on thedesign and optimization of multifunction coils are provide below.

FIG. 26A illustrates a system configured for producing a multifunctioncoil operated to generate multiple types of magnetic fields, inaccordance with some embodiments. The exemplary system schematicallydepicted in FIG. 26A comprises a controller 2675 coupled to a coil 2600to cause the coil to generate at least a gradient magnetic field and anRF magnetic field. According to some embodiments, controller 2675comprises gradient amplifier 2620 coupled to coil 2600 via low passfilter 2630. In operation, a console 2685 may issue gradient commandinput 2610 to cause gradient amplifier 2620 to drive coil 2600 toproduce one or more gradient fields in accordance with a desired pulsesequence (e.g., a pulse sequence designed to acquire MR data for use inproducing one or more images). In this manner, coil 2600 can be operatedas a gradient coil (e.g., Gx, Gy, etc.), for example, in a low-field MRIsystem.

Controller 2675 further comprises RF amplifier 2650 coupled to coil 2600via high pass filter 2640. Console 2685 may also issue RF command input2660 to cause RF amplifier 2650 to drive coil 2600 to produce RFmagnetic fields in accordance with the desired pulse sequence. By doingso, coil 2600 can also be operated as an RF coil. Controller 2675 may,according to some embodiments, also utilize coil 2600 to detect MRsignals emitted in response to the RF magnetic fields generated by coil2600 so that coil 2600 can be operated as a RF transmit coil and an RFreceive coil. For example, FIG. 26B illustrates a multifunction coil2600 driven by controller 2675 with both a transmit path 2680 andreceive path 2690 to enable use of multifunction coil 2600 as both atransmit and receive coil. T/R switch 2687 switches between transmitpath 2680 and receive path 2690 to allow multifunction coil to beselectively operated to produce RF magnetic fields and to detect MRsignals emitted in response to an RF transmit cycle.

It should be appreciated that coil 2600 may be used as an RF receivecoil with or without also operating coil 2600 as an RF transmit coil andvice versa. Thus, controller 2675 is configured to operate coil 2600 asboth a gradient coil and an RF coil so that coil 2600 can providemultiple functions in an MRI system, such as a low-field MRI system. Itshould be appreciated that the controller illustrated in FIGS. 26A and Bis merely exemplary and may include further components and/or mayexclude one or more of the components illustrated, as a suitablecontroller for implementing a multifunction coil may include anycombination of components configured to cause a coil to generatemultiple types of magnetic fields.

According to some embodiments, a multifunction coil (e.g., coil 2600) isoperated as a Gx gradient coil and as an RF transmit/receive coil.According to some embodiments, a multifunction coil is operated as a Gygradient coil and as an RF transmit/receive coil. It should beappreciated that more than one multifunction coil may be utilized in anMRI system. For example, according to some embodiments, a firstmultifunction coil is configured to operate as a Gx gradient coil and asecond multifunction coil is configured to operate as a Gy gradientcoil, with both first and second multifunction coils also operating asRF transmit/receive coils. Multiple multifunction coils operated in thismanner can be used to implement multiple transmit/receive channels thatcan be used to improve SNR, reduce acquisition times, or both. Forexample, MR data obtained from multiple receive coils may be combined toincrease SNR. When both Gx and Gy gradient coils are also used asreceive coils, a 90 degree phase difference will exist between therespective receive channels (i.e., because the Gx and Gy gradient coilsare substantially orthogonal to one another as well as substantiallyorthogonal to the B0 magnetic field). This quadrature relationship canbe exploited to boost SNR by as much as the square root of two.Alternatively, or in addition to increasing SNR, multipletransmit/receive coils may be used to perform parallel MR to reduce theacquisition time needed to obtain MR data for generating one or moreimages.

FIG. 27 illustrates a system for providing a multifunction coil inconnection with a specific configuration for a gradient coil set. Itshould be appreciated that while the gradient coil set illustrated inFIG. 27 is labeled as a Gx gradient coil set, this is not a limitationas the same techniques can equally be applied to a Gy gradient coil set.In FIG. 27 , the gradient coil set is configured as coil pairs, witheach pair having coils connected with opposite polarity, or using 180degree in-line phase shifter circuits, such that they are driven 180degrees out of phase. In FIG. 27 , exemplary controller 2775 isconfigured to utilize the gradient coil set to also operate as an RFcoil to transmit and/or receive RF magnetic fields. According to someembodiments, the gradient coil set is operated as a single RF coil. Onetechnique to achieve this is to treat the gradient coil set as a singlecontinuous coil by coupling, in addition to each high pass filter, arespective balun to a 1:4 RF splitter/combiner. In this way, a gradientcoil set of the configuration illustrated in FIG. 27 can also be drivenas a transmit and/or receive coil. Alternatively, each coil in thegradient coil set can be treated separately from the RF perspective bydriving each coil with a respective RF amplifier and high pass filter sothat the gradient coil set can be operated as four separate transmitand/or receive coils.

The inventors have appreciated that multifunction coil techniques mayfacilitate reduced cost and/or reduced size low-field MRI systems. Forexample, techniques described herein can be applied to the low-field MRIsystems for imaging the head illustrated in FIGS. 22A-C of the '652Application. These systems include a head component (e.g., a helmet)configured to accommodate the head of the person being imaged. The headcomponent may have incorporated therein one or more coils of thelow-field MRI system (e.g., a B0 magnet, one or more gradient coils, oneor more transmit/receive coils, etc.). The inventors have recognizedthat the illustrated head imaging systems can be produced with at leastone coil incorporated or housed in the head component that is configuredto produce at least two types of magnetic fields (i.e., the headcomponent can house one or more multifunction coils). According to someembodiments, the head component comprises a coil configured to transmitand/or receive RF magnetic fields and to generate at least one gradientmagnetic field. As discussed in the foregoing, such a multifunction coilmay be achieved by coupling a controller to the multifunction coil tooperate the coil as both an RF coil and a gradient coil (e.g., bycoupling a first amplifier and high pass filter to the coil to drive thecoil to generate and/or receive RF magnetic fields and coupling a secondamplifier and a low pass filter to the coil to drive the coil togenerate at least one gradient magnetic field). In this manner, one ormore multifunction coils may be utilized to generate both transmit RFpulses and gradient pulses in accordance with a desired pulse sequence,and detect MR signals emitted in response.

By utilizing the above described techniques to implement a multifunctioncoil, the cost of the resulting system may be reduced as a single coilcan be used to produce more than one type of magnetic field for the MRIsystem. Additionally, a multifunction coil can reduce the footprint ofthe system and/or facilitate designs where the space available forincorporating the system's magnetics is limited (e.g., in the headimaging systems discussed above). Another benefit of some embodimentsdescribed above relates to the ability to implement multiple transmitand/or receive channels using the gradient coils of the MRI system.

The inventors have appreciated that the optimization techniquesdescribed herein may be applied to generally optimize the configurationof a multifunction coil. As discussed above, an optimization can beformulated that determines a coil configuration that meets one or moreconstraints and that, when simulated, produces a magnetic field thatsatisfies one or more criteria. By formulating an optimization toinclude regularization terms for both gradient and RF coils, a coilconfiguration can be determined that can produce both gradient and RFmagnetic fields that meet specified criteria. Thus, the optimizationtechniques described herein can be applied to produce single functionand multifunction coils alike.

U.S. patent application Ser. No. 14/845,949 ('949 Application), filedSep. 4, 2015 and entitled “Noise Suppression Methods and Apparatus”describes, among other subject matter, techniques for using auxiliarysensors to facilitate characterization of the noise environment of alow-field MRI system to suppress noise received by one or more RFreceive coils, which application is herein incorporated by reference inits entirety. The techniques described in the '949 Application allow amagnetic resonance imaging system (e.g., a low-field MRI system) to beoperated outside a shielded room, facilitating production of MRI systemsthat can be operated in arbitrary environments so that MRI can be usedin numerous situations where conventional MRI cannot. Any of the noisecancellation techniques described in the '949 Application can be used inconnection with coil configurations described herein. Moreover, theinventors have appreciated that the optimization techniques describedherein can also be applied to determine an optimal configuration of oneor more auxiliary sensors (e.g., an auxiliary coil) for use in noisesuppression. In particular, one or more criteria and/or one or moreconstraints corresponding to desired operation of an auxiliary coil maybe incorporated into the optimization scheme described in the foregoingto determine a coil configuration for the auxiliary coil. As furtherdiscussed in the '949 Application, some embodiments include using a RFcoil both as an auxiliary coil and as a primary coil and, in thisrespect, represents another example of a multifunction coil. Theoptimization techniques described herein may also be used to determine aconfiguration for a multifunction coil configured to operate both as aprimary and auxiliary coil or additionally as a gradient coil as well.

As discussed above, optimization techniques described herein may be usedto optimize a configuration for a head coil disposed on a surface of ahelmet adapted to accommodate a patient's head. The inventors haveappreciated that one or more auxiliary coils may be positioned on orproximate the helmet to facilitate noise suppression. For example, thehead coil may be configured to optimally detect MR signals emitted fromthe patient within a field of view located within the helmet. One ormore auxiliary coils may be positioned proximate (or on) the helmet sothat it responds to the noise environment but does not respond to MRsignals emitted from the field of view. The noise signal from the one ormore auxiliary coils may be used to suppress noise in the MR signalsdetected by the head coil, for example, using any of the techniquesdescribed in the '949 Application.

As discussed above, and in detail in the '949 Application, one or moreauxiliary coils may be used to detect the noise environment but not MRsignals emitted from the field of view of an MRI system. This istypically achieved by positioning one or more auxiliary coils proximatea primary coil (e.g., the main receive coil of the MRI system) so thatthe auxiliary coil is responding to as similar a noise environment asthe primary coil as possible, yet is located outside the detection rangeof emitted MR signals so that the auxiliary coil does not respond toemitted MR signals. In this manner, the one or more auxiliary coilscharacterizes substantially the same noise environment as the primarycoil, but does not respond to MR signals so that the noise environment,as characterized by the one or more auxiliary coils, can be used tosuppress noise detected by the primary coil. However, when positionedproximate one another in this manner, the primary coil and the auxiliarycoil may inductively couple such that the one or more auxiliary coilshas a response to MR signals emitted from the field of view even thoughit is outside the range of the MR signals because of the inductivecoupling with the primary coil. Because the auxiliary coil responseincludes MR signal content as well, the described noise suppressiontechniques will operate to suppress the MR signal content detected bythe primary coil, thereby reducing SNR instead of increasing SNR asintended.

The inventors have appreciated that the optimization techniquesdescribed herein may be utilized to generate a configuration for anauxiliary coil that reduces or eliminates inductive coupling with theprimary coil. Using this technique, an auxiliary coil can be positionedproximate a primary coil while avoiding deleterious inductive coupling.According to some embodiments, a configuration of one or more auxiliarycoils is optimized to reduce or eliminate inductive coupling with aprimary coil. For example, the optimization scheme may incorporate oneor more terms that define a region over which the auxiliary coil issensitive to noise, which region excludes the region where MR signalscan be detected directly, and one or more terms that operate to minimizeinductive coupling between one or more auxiliary coils and a primarycoil (e.g., one or more terms that cause the resulting configuration to,when operated in conjunction with the primary coil, suppress or cancelmutual inductance between coils). According to some embodiments, aconfiguration for a primary coil and one or more auxiliary coils can beoptimized together so that the resulting primary coil has generallyoptimal performance with respect to specified criteria for receive coiloperation and the resulting one or more auxiliary coils operates withminimal or no inductive coupling with the primary coil.

It should be appreciated that the techniques described herein can beapplied to determine a coil configuration optimized for any portion ofthe human anatomy and the illustrated head coils are merely exemplary.In particular, the optimization techniques described herein are agnosticwith respect to the particular surface on which a coil configuration isoptimized. As such, the techniques described herein can be applied toany surface that can be modeled. For example, using a triangular mesh tomodel the surface, virtually any surface can be triangulated and, assuch, there are no meaningful limitations on the geometry of an RF coilto which these techniques can be applied. Accordingly, a configurationfor RF coils for any portion of the anatomy can be determined usingtechniques described herein, including, but not limited to head coils,coils for the torso, arms, legs, hands, feet, etc., or any combinationthereof. In addition, the optimization techniques can be applied to anycombination of multifunction coils for any desired part of the anatomy.

Having thus described several aspects and embodiments of the technologyset forth in the disclosure, it is to be appreciated that variousalterations, modifications, and improvements will readily occur to thoseskilled in the art. Such alterations, modifications, and improvementsare intended to be within the spirit and scope of the technologydescribed herein. For example, those of ordinary skill in the art willreadily envision a variety of other means and/or structures forperforming the function and/or obtaining the results and/or one or moreof the advantages described herein, and each of such variations and/ormodifications is deemed to be within the scope of the embodimentsdescribed herein. Those skilled in the art will recognize, or be able toascertain using no more than routine experimentation, many equivalentsto the specific embodiments described herein. It is, therefore, to beunderstood that the foregoing embodiments are presented by way ofexample only and that, within the scope of the appended claims andequivalents thereto, inventive embodiments may be practiced otherwisethan as specifically described. In addition, any combination of two ormore features, systems, articles, materials, kits, and/or methodsdescribed herein, if such features, systems, articles, materials, kits,and/or methods are not mutually inconsistent, is included within thescope of the present disclosure.

The above-described embodiments can be implemented in any of numerousways. One or more aspects and embodiments of the present disclosureinvolving the performance of processes or methods may utilize programinstructions executable by a device (e.g., a computer, a processor, orother device) to perform, or control performance of, the processes ormethods. In this respect, various inventive concepts may be embodied asa computer readable storage medium (or multiple computer readablestorage media) (e.g., a computer memory, one or more floppy discs,compact discs, optical discs, magnetic tapes, flash memories, circuitconfigurations in Field Programmable Gate Arrays or other semiconductordevices, or other tangible computer storage medium) encoded with one ormore programs that, when executed on one or more computers or otherprocessors, perform methods that implement one or more of the variousembodiments described above. The computer readable medium or media canbe transportable, such that the program or programs stored thereon canbe loaded onto one or more different computers or other processors toimplement various ones of the aspects described above. In someembodiments, computer readable media may be non-transitory media.

The terms “program” or “software” are used herein in a generic sense torefer to any type of computer code or set of computer-executableinstructions that can be employed to program a computer or otherprocessor to implement various aspects as described above. Additionally,it should be appreciated that according to one aspect, one or morecomputer programs that when executed perform methods of the presentdisclosure need not reside on a single computer or processor, but may bedistributed in a modular fashion among a number of different computersor processors to implement various aspects of the present disclosure.

Computer-executable instructions may be in many forms, such as programmodules, executed by one or more computers or other devices. Generally,program modules include routines, programs, objects, components, datastructures, etc. that perform particular tasks or implement particularabstract data types. Typically the functionality of the program modulesmay be combined or distributed as desired in various embodiments.

Also, data structures may be stored in computer-readable media in anysuitable form. For simplicity of illustration, data structures may beshown to have fields that are related through location in the datastructure. Such relationships may likewise be achieved by assigningstorage for the fields with locations in a computer-readable medium thatconvey relationship between the fields. However, any suitable mechanismmay be used to establish a relationship between information in fields ofa data structure, including through the use of pointers, tags or othermechanisms that establish relationship between data elements.

The above-described embodiments of the present invention can beimplemented in any of numerous ways. For example, the embodiments may beimplemented using hardware, software or a combination thereof. Whenimplemented in software, the software code can be executed on anysuitable processor or collection of processors, whether provided in asingle computer or distributed among multiple computers. It should beappreciated that any component or collection of components that performthe functions described above can be generically considered as acontroller that controls the above-discussed function. A controller canbe implemented in numerous ways, such as with dedicated hardware, orwith general purpose hardware (e.g., one or more processor) that isprogrammed using microcode or software to perform the functions recitedabove, and may be implemented in a combination of ways when thecontroller corresponds to multiple components of a system.

Further, it should be appreciated that a computer may be embodied in anyof a number of forms, such as a rack-mounted computer, a desktopcomputer, a laptop computer, or a tablet computer, as non-limitingexamples. Additionally, a computer may be embedded in a device notgenerally regarded as a computer but with suitable processingcapabilities, including a Personal Digital Assistant (PDA), a smartphoneor any other suitable portable or fixed electronic device.

Also, a computer may have one or more input and output devices. Thesedevices can be used, among other things, to present a user interface.Examples of output devices that can be used to provide a user interfaceinclude printers or display screens for visual presentation of outputand speakers or other sound generating devices for audible presentationof output. Examples of input devices that can be used for a userinterface include keyboards, and pointing devices, such as mice, touchpads, and digitizing tablets. As another example, a computer may receiveinput information through speech recognition or in other audibleformats.

Such computers may be interconnected by one or more networks in anysuitable form, including a local area network or a wide area network,such as an enterprise network, and intelligent network (IN) or theInternet. Such networks may be based on any suitable technology and mayoperate according to any suitable protocol and may include wirelessnetworks, wired networks or fiber optic networks.

Also, as described, some aspects may be embodied as one or more methods.The acts performed as part of the method may be ordered in any suitableway. Accordingly, embodiments may be constructed in which acts areperformed in an order different than illustrated, which may includeperforming some acts simultaneously, even though shown as sequentialacts in illustrative embodiments.

All definitions, as defined and used herein, should be understood tocontrol over dictionary definitions, definitions in documentsincorporated by reference, and/or ordinary meanings of the definedterms.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Multiple elements listed with“and/or” should be construed in the same fashion, i.e., “one or more” ofthe elements so conjoined. Other elements may optionally be presentother than the elements specifically identified by the “and/or” clause,whether related or unrelated to those elements specifically identified.Thus, as a non-limiting example, a reference to “A and/or B”, when usedin conjunction with open-ended language such as “comprising” can refer,in one embodiment, to A only (optionally including elements other thanB); in another embodiment, to B only (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

Also, the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having,” “containing,” “involving,” andvariations thereof herein, is meant to encompass the items listedthereafter and equivalents thereof as well as additional items.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” “composed of,” and the like are tobe understood to be open-ended, i.e., to mean including but not limitedto. Only the transitional phrases “consisting of” and “consistingessentially of” shall be closed or semi-closed transitional phrases,respectively.

1-20. (canceled)
 21. A radio frequency component configured for magneticresonance imaging (MRI), the radio frequency component comprising: oneor more coils configured to generate magnetic field components, the oneor more coils including a first conductor arranged in a plurality ofturns having non-uniform spacing; wherein the one or more coils aretuned to resonate at a frequency corresponding to a B₀ field having afield strength of less than or equal to 0.2 T.
 22. The radio frequencycomponent of claim 21, wherein the one or more coils comprises a firstcoil configured to generate the magnetic field components, and a secondcoil configured to be responsive to magnetic resonance signalcomponents.
 23. The radio frequency component of claim 21, wherein theone or more coils comprises a first coil configured to generate themagnetic field components, and wherein the first conductor is part ofthe first coil.
 24. The radio frequency component of claim 21, whereinthe one or more coils comprises a first coil and a second coil, thefirst coil and the second coil tuned to resonate at the frequencycorresponding to the B₀ field, and wherein the field strength is lessthan or equal to 0.1 T and greater than or equal to 50 mT.
 25. The radiofrequency component of claim 21, wherein the plurality of turns includes5 turns.
 26. The radio frequency component of claim 21, wherein the oneor more coils comprises a first coil, and wherein a configuration of thefirst coil is determined by performing at least one optimization thatdetermines a value for at least one parameter of a model of the radiofrequency component.
 27. The radio frequency component of claim 26,wherein the nonuniform spacing is determined based, at least in part, onthe at least one optimization.
 28. The radio frequency component ofclaim 26, wherein a number of turns in the plurality of turns isdetermined based, at least in part, on performing the at least oneoptimization using the model of the radio frequency component.
 29. Theradio frequency component of claim 21, wherein a first coil of the oneor more coils is tuned to resonate at a target frequency, and wherein anumber of turns in the plurality of turns are limited so that aself-resonance of the first conductor is at a frequency at least twicethe target frequency.
 30. The radio frequency component of claim 21,wherein the first conductor is arranged in a three-dimensional geometryabout a region of interest in accordance with a coil configuration ofthe first conductor in the three-dimensional geometry.
 31. The radiofrequency component of claim 30, wherein the magnetic field components,generated by a first coil of the one or more coils, cause a magneticresonance response from the region of interest.
 32. The radio frequencycomponent of claim 21, wherein each of the plurality of turns comprisesa conductive path provided 360° or substantially 360° about a referenceaxis.
 33. The radio frequency component of claim 21, wherein the firstconductor has a length of at least 1 meter.
 35. An apparatus,comprising: a radio frequency component configured for magneticresonance imaging (MRI), the radio frequency component comprising one ormore coils configured to generate magnetic field components, the one ormore coils including a first conductor arranged in a plurality of turnshaving non-uniform spacing; and a support structure configured toaccommodate a portion of a body of a patient, the support structurehaving grooves configured to accommodate the first conductor.
 36. Theapparatus of claim 35, wherein the first coil is configured to generatemagnetic field components to cause a magnetic resonance response from aregion of interest.
 37. The apparatus of claim 35, wherein the one ormore coils comprises a first coil and a second coil, and wherein aconfiguration of the first coil is determined by performing at least oneoptimization that determines a value for at least one parameter of amodel of the radio frequency component.
 38. The apparatus of claim 35,wherein the radio frequency component is a head coil, and wherein thesupport structure comprises a helmet formed to accommodate a human head.39. The apparatus of claim 35, wherein the one or more coils comprises afirst coil configured to generate the magnetic field components, and asecond coil configured to be responsive to magnetic resonance signalcomponents, and wherein the first conductor is part of the first coil.40. The apparatus of claim 35, wherein the one or more coils are tunedto resonate at a frequency corresponding to a B₀ field having a fieldstrength of less than or equal to 0.2 T.